Okay, so let's say you have a toy that you really like to play with. But your friend tells you that the person who made the toy was mean to other kids and did bad things. You might decide that you don't want to play with that toy anymore because you don't want to support bad behavior.
Boycotting Israel in sports is kind of like that. Some people believe that Israel does bad things to other people, especially to the people of Palestine. So they don't want to support Israel by playing sports with them or watching their games.
It's kind of like saying, "I don't want to be friends with the kid who is mean to others." But instead of a kid, it's a whole country. And instead of being friends, it's about playing sports with them.
It can be a complicated issue, and people have different opinions on it. But that's the basic idea behind it.