Okay, so you know how sometimes you get a little sore spot in your tummy that hurts when you eat or drink? That's kind of like what an ulcer is for monkeys. Except instead of being caused by eating too much candy or spicy food, it can happen to monkeys who are in charge of things - like the boss monkeys.
When monkeys are in charge, they have a lot of important decisions to make and they can get stressed out from all of the responsibilities. Just like when you get nervous before a big test or presentation, the monkeys can get really anxious and their bodies can make too much acid in their tummy. That extra acid can start to hurt the lining of their stomach and cause an ulcer.
It's important for the monkeys to get help from doctors and maybe even take medicine to help their tummy feel better. And even more importantly, it's good for the other monkeys to be nice to the boss monkey and help them out with their important decisions so they don't have to do it all by themselves and get too stressed out.