What is that curved thing on top of the bow called

First you have to know what kind of bow you are asking about.

Traditional bows generally consist of a bow handle and a bow blade , while traditional Chinese angle bows or laminated other material bow components are the bow tip, bow arm, bow handle.

Recurve bows consist of a bow handle, a bow blade, an arrow platform, a sight, a side pad, and a balance bar.

Compound bows are made up of pulleys to save energy and also include all the names of the recurve bow except for the side pads.