€STR stands for Euro Short-Term Rate. It is a way of keeping track of how much money it costs to borrow and lend money in Europe.
You know how sometimes if you want to buy a toy or a piece of candy, you might have to ask your parents to lend you some money? Well, grown-ups and companies also sometimes need to borrow money from other grown-ups and companies to buy big things like houses or buildings or to do business with each other.
When someone borrows money, they usually have to pay back more than they borrowed, because the person who lent them the money wants to make some extra money too. How much extra money they have to pay back is called the interest rate.
€STR is just one of many different interest rates you might hear about. It's a special one that's used in Europe, and it's for really short periods of time - usually just one day or a few days.
The people who keep track of €STR are called the European Central Bank. They use the €STR to help figure out how well the European economy is doing and to make sure that borrowing and lending is working like it should.
So when you hear grown-ups talking about €STR, you can impress them with your knowledge and say "Oh, that's the interest rate for short-term borrowing and lending in Europe!"