Question

what does it mean "when two events A and B are mutually exclusive"?

Answer 1

It means that elements of set A have nothing in common with elements of set B

Answer 2

When the events A and B are mutually exclusive it's impossible for both A and B to happen, so we happen upon this nice shortcut: (probability A or B happens) = (probability of A) + (probability of B) ... Event A and Event "not A" are mutually exclusive: we can't have A happen and not happen at the same time.