What does mutually exclusive mean?

It means that one thing can not happen together with another thing.

It's always an EITHER...OR thing, but never both. If you flip a coin, you get EITHER heads OR tails.

If you select 4 different numbers out of 10, and you've selected your first number, you cannot select that number again. In formula: 1st number 10 choices, 2nd number 9 choices, etc., or Choosing 4 out of 10: ##10*9*8*7## possibilities. (if you could choose the same number again it would be ##10^4##