back rank weakness

What makes a back rank weak?

No escape square. Opponent can attack back rank with more pieces than I can protect.

There seems to be a lot more, but as I'm studying this, it seems people say things like the back rank is weak or you can create a weakness without defining the term.

Thanks

The bank rank is weak if the threat of a bank rank checkmate is the source of an advantage for the other player.. So a player might use the weakness as the basis of a tactic and gain a material advantage by causing the other player to sacrifice material in order to prevent checkmate, or they may use the weakness to gain a positional advantage as the player with the weakness may be restricted in the moves they can make so as to prevent mate. If you cant meet your opponents threats due to the risk of back rank mate, then you have a back rank weakness.