The union A ∪ B is the event "A or B (or both)". It contains all outcomes that are in A, in B, or in both.
If A = {2, 4, 6} (even) and B = {3, 6} (multiple of 3), then A ∪ B = {2, 3, 4, 6}.
Notice 6 appears in both A and B, but you only list it once in the union. Next unit shows how to calculate P(A ∪ B).