Compound interest
Compound interest in the context of maths at school usually means the following: if you have money in a bank account the bank usually pays you a yearly interest. Say you have 200 Euro in your account and the bank pays you an interest of 10 percent every year. Then you get 20 Euro every year from the bank. That money is called the interest. If the interest, however, goes straight to your bank account, it then becomes part of your savings. The second year your interest from the first year will then itself add 2 Euros to the overall interest. That is called a compound interest. The overall interest you get from the bank in the second year would then be 22 Euros.
In German compound interest is often translated by the plural "Zinseszinsen" rather than the singular "Zinseszins" although both will be understood.