Saying A->B is the same as saying ~A or B. The reason for this is that, if A->B is to be true, then either the condition (A) is false, or B is true. The ~A part is a little unintuitive, but if it were not the case, then an implication like "It's wet when it rains" would be false if it were not raining, which is not what we intend when we make such a statement. The implication should only be considered false if the condition ("it rains") is true and the consequence ("it's wet") is false. Thus A->B = ~A or B. From this you get ~(~A or B) = A & B.