To reimburse someone means to pay them back money they spent on your behalf or for something you're responsible for. When your parents reimburse you for buying supplies for a school project with your own money, they're giving you back exactly what you spent.

Reimbursement happens all the time in the adult world. If a teacher buys classroom supplies with her own money, the school might reimburse her later. If someone travels for work and pays for their own hotel, their company reimburses them when they return. A doctor's office might reimburse a patient who accidentally paid twice for the same visit.

The key idea is that someone pays first, and then gets their money back. It's different from a loan: when you're reimbursed, you're not being given extra money or being paid for work. You're simply getting back what you already spent. Think of it like this: if you pay for your friend's lunch and they pay you back that same five dollars later, they've reimbursed you.