Hallel is a special prayer that Jewish people say during some holidays, like Passover and Hanukkah. It's a way to celebrate and thank God for all the good things in life.
It's like when you get a really good grade on your test, and your parents take you out for ice cream to celebrate. Except instead of ice cream, Jewish people sing prayers to show their appreciation!
Hallel is made up of six different prayers, and each one is a little different. Some parts are really happy and cheerful, like when you get to do something fun with your friends. Other parts are more serious, like when you talk to your parents about something that's bothering you.
Overall, Hallel is a way for Jewish people to come together and praise God for everything good in life.