You're going to read the following terms used by coaches here on this site.  It's not hard but not knowing what they're referring to can render an important post meaningless.  There are three basic terms used by all coaches.  Since most of us think about offense, I'll start with how offensive coaches identify running lanes.  They do so by using LETTERS to identify the run holes.  Here are the gaps as lettered against a wishbone formation:

                              D    C    B  A  A  B    C    D
                                    O  O  O  0  O  O  O
                                                    O
                                                    O
                                          O                  O

So the "A" gaps are between the guard and center, the "B" gaps between the guard and tackle, and the "C" gaps are between the TE and Tackles.  The furthest outside gap is "D" (outside the TE).

Some coaches will use words instead of letters.  "Iso" is "B" gap.  "Off tackle" is "C" gap and "Sweep" is "D" gap.

Defensive coaches don't use letters.  They use numbers (called "Techs").  A SINGLE digit tech is used for a down lineman.  Here are the numbers versus a Double Wing:

                        9  8    6 5 4 3 2 1 0 1 2 3 4 5 6  8  9   
                                    O  O    O    0  O    O  O
                              O                      O                    O
                                                      O
                                                           
If you'll notice, you'll see two things.  First, all odd numbers are in the gaps (The "A", "B", "C", and "D" gaps) and all even numbers are "head up" on an offensive player.

Second, you'll notice there is no #7.  There actually is a #7 tech.  I just didn't include it.  The term "5" is used to describe a defender lined up on the outside shoulder of the offensive tackle (OT).  In most cases, that would place the "5" tech defender on the inside shoulder of the TE.  But some crafty coaches (like me) widen the split of the Tight End (TE).  In that case, a "5" tech may be on the outside shoulder of the OT but not on the inside shoulder of the TE.  Here's how that problem is solved using my DC Wing T formation:

                        7    5                  5 7       
                    O          O  O 0 O  O    O
                                        O                            O
                                    O
                                        O

A "5" describes a defender on the outside shoulder of the OT and a "7" describes the inside shoulder of an End. While that may, at first, sound like the same spot, the above shows it is different.  My famous DC46 uses "7" techs" and not "5's".

When linebacker positions are referred to, their tech is multiplied by 10:

                      40    00 30

            O        O  O 0 O  O    O
                              O                        O
                        O    O 

So an "even tech" LB over the LT above is a "40" and an even tech LB over the center is a "00" and an LB aligned in the guard/tackle gap becomes a "30".

Their is another term used here by coaches called COVERAGE.  Coverage refers to how you defend the pass.  It can be MAN or ZONE.  But, if you remove the last three letters from COVERAGE you get this:

COVER

The term "COVER" is not the same as COVERAGE.  "COVER" refers to how many DEEP receivers you can cover defensively.  In most cases, the most you can cover is four receivers and the least you can cover is zero and it is determined by the number of third row defenders.  If you go to pro football sites you can find terms like "Cover 7" but you won't find them here.  "Cover 7", in the words of Admiral James T Kirk, is a term used by a coach who "has done too much LDS".

Here is my famous DC46 in COVER 0:

                        W    F    M              C
              C  E    T  N    T    S    E   
            O        O  O 0 O  O    O
                              O                        O
                        O    O 

Notice, there are no deep pass defenders.  COVER 0 is usually used at the goal line as a defense or all over the field versus an 8 year old offense as they usually can't pass.

Here is the same defense in Cover 1:

                              F
                        W            M            C
              C  E    T  N    T    S      E   
            O        O  O 0 O  O    O
                              O                        O
                        O    O 

F is able to cover ONE receiver deep.  Being deep already, he can easily find and cover ONE RECEIVER running deep.  Some call this a "high safety", particularly if he's unusually deep (The letter F refers to "free safety").

Here is a COVER 2 defense against my well known DC Wing T:

                        WS              SS
            C              B    B                    C
                  E  T      N    T        E
            O        O  O 0 O  O    O
                              O                        O
                        O    O 

The two deep defenders are WS (Weak Safety) and SS (Strong Safety).  Normally any safety covering a TE is called a STRONG safety and any safety covering a Split End is called a WEAK safety.  However, I wouldn't suggest you name your safeties that as your weak safety might be insulted.

Here is a COVER 3 defense against the same DC Wing T offense:

          C                  S                        C
                  B          B B                B                     
                    E    T        T    E
            O        O  O 0 O  O    O
                              O                        O
                        O    O 

Cover 4 is pretty unusual but my infamous Killer Bee defense uses Cover 4, making it the ideal defense to cover a four receiver spread offense.

Occasionally, you will hear of "Inverted Cover 3".  This a Cover 3 defense with the free safety (F) up closer to help stop the run:

            C                                            C
                              F
                        B            B
                  E    T  G    G  T        E
            O        O  O 0 O  O    O
                              O                        O
                        O    O 

This basic information should get you through most posts with a successful understanding or, if you wish to post your own question, you'll know how to describe the alignment.

Have fun!  🙂