Ameinu is a special word that we say at the end of prayers to show that we agree with what has been said. It's like when someone says something and you nod your head to show that you understand or agree.
When someone finishes saying a prayer, we say ameinu to show that we believe in what was prayed for. For example, if someone says a prayer asking for help for sick people, we say ameinu to show that we want sick people to get better.
It's a simple way to show our faith and support in what we believe in. So, whenever you hear someone say ameinu, remember that they are showing agreement and support for the prayers that were said.