An understudy is like a backup dancer or singer but for a play or musical. When someone is going to be in a play or musical, they practice a lot to make sure they do a really good job. But sometimes, things happen and the person can't be there to perform in the play. Maybe they get sick or have to go somewhere else. That's where the understudy comes in! The understudy is someone who has been practicing too, just in case something happens to the main performer.
So, imagine you are going to see a play called "The Lion King". The person who plays Simba is the main performer. But if they get sick, the understudy who has been practicing how to play Simba will be able to take their place. This way, the show can go on and everyone can still have a great time!
Being an understudy is very important because if there is no one to replace the main performer, the play or musical would have to be cancelled. So, they make sure to always have a backup just in case!