Okay kiddo, have you ever heard the phrase "cui bono"? It's a Latin phrase that means "who benefits?" and it's used to figure out why somebody might do something. So when we want to know why something happened, we ask "cui bono?" to find out who benefits from it.
For example, let's say that somebody stole a cookie from the cookie jar. We might ask "cui bono?" to figure out who had a reason to steal the cookie. Did the person who stole the cookie benefit from it, or did somebody else benefit from it?
When people use "cui bono" in a more serious way, they're usually trying to figure out why something happened that affects a lot of people. For example, if a big company closes down and a lot of people lose their jobs, we might ask "cui bono?" to figure out who is benefiting from the company closing down. Is it the company's owners who are getting rich, or is it somebody else who is benefiting from the company closing?
So when somebody says "cui bono," they're really just trying to find out who is gaining something from a situation. It's like being a detective and trying to solve a mystery by figuring out who had a motive.