Events A and B are mutually exclusive if they cannot both happen. Their intersection is empty: A ∩ B = ∅.
Rolling a 2 and rolling a 5 are mutually exclusive. You cannot roll both on one die. P(A ∩ B) = 0.
For mutually exclusive events, the addition rule simplifies: P(A ∪ B) = P(A) + P(B).