Repossession is when someone takes away something that belongs to you because you didn't pay for it or didn't follow the rules of the agreement you made with them when you got it.
It's like, if you borrow a toy from your friend and they tell you that you have to give it back by a certain time or else they'll take it away. If you don't give it back on time, they might come and take it from you so they can have it back.
But with repossession, it's usually for things like cars or houses that people bought on credit or loans, and then fell behind on their payments. When that happens, the lender can take the car or house back because they still technically own it until you completely pay for it.
Repossession can be really stressful and sad for the person who loses the thing they couldn't pay for, but it's also a way for the lender to protect their investment and make sure they get their money back in some way.