As the sibling comment put it so succinctly, "proliferation". To expand upon that, natural selection basically says that traits which are beneficial to survival

As the sibling comment put it so succinctly, "proliferation". To expand upon that, natural selection basically says that traits which are beneficial to survival or reproduction will become more widespread in a population over time, and traits which are detrimental to survival or reproduction will become more rare in a population over time. Ergo, the fact that ADHD is rather widespread indicates that it is, on the whole, beneficial to the survival and/or reproduction of the individual that carries it. Of course, this is a drastic simplification. If you want to dive deeper, I suggest reading "The Selfish Gene".