Okay, so a subpoena is like a grown-up version of your mommy or daddy telling you that you have to do something. Except it's not your mommy or daddy, it's someone else who has the power to make you do something.
Let's say you have a toy that someone else wants. They can't just take it from you because that's not fair. But they can ask a special person called a judge to help them get it back. The judge can give them a piece of paper called a subpoena that says you have to go to court and give the toy back.
Or maybe you saw something happen that the judge really needs to know about. The judge can also use a subpoena to make you come to court and tell them exactly what you saw. It's like a really important show-and-tell.
So a subpoena is a special piece of paper that gives someone the power to make you do something, like go to court, share information, or give something back. And if you don't do what the subpoena says, you could get in trouble just like you would if you didn't listen to your mommy or daddy.