Okay kiddo, so Exercise Tiger was like a big pretend fight that some soldiers had during World War II. It was a way to practice for a real battle so they would be ready.
They went to a beach in England to do the pretend fight. Lots of boats and tanks and soldiers were there. But something bad happened.
The soldiers didn't know that some German boats were nearby and they saw all the pretend fighting. So the German boats shot at the soldiers and some of them got hurt or died.
It was a really sad day and the soldiers felt very bad about what happened. But they learned some important things from that day so they could be better prepared for the real war.