So, imagine you and your friend are playing a game and you're losing really badly. You're getting frustrated and you don't think you can win anymore. So, you decide to give up and say "I quit, you win." That's kind of like what capitulation is.
Capitulation is when one side in a big conflict, like a war or a battle, decides they can't win and gives up. They sign a treaty, which is like an agreement, with the other side that says they'll stop fighting and give up their land or power.
It's not a fun thing to have happen because it usually means the losing side has to give up a lot of things and the winning side gets what they want. But it's better than fighting forever and ever and never ending the conflict. It's like saying "let's just stop and move on."