A cartouche is a name plate. It's usually oval with your name written in the middle of it. A cartouche is attached to your coffin.
The ancient Egyptians wanted to make sure that their two souls - the Ba and the Ka - could find their way back to their tomb at night, after they died. No one wanted their Ba or Ka to get lost.
A cartouche made it very easy for a Ba and Ka to find their way home.