Okay kiddo, "ac complexity" refers to how fast a computer can solve a problem. Imagine you have a big puzzle that you want to solve. The ac complexity is like how fast you can put that puzzle together.
Now, some puzzles are easy and you can solve them quickly, but some puzzles are really hard and can take a really long time to solve. The ac complexity measures how long it takes a computer to solve those harder puzzles.
Not all problems can be solved quickly, even by a computer. That's why we use special methods to try to solve them as quickly as possible. The ac complexity helps us understand which methods we should use for different problems, so we can solve them faster.
So, sometimes when people talk about ac complexity, they are really talking about how hard a problem is to solve. The harder the problem, the higher the ac complexity. But don't worry, even if a problem has a high ac complexity, there are always ways to solve it, it just might take a little longer!