A chevet is a fancy French word for the part of a church where the altar and the choir stalls are located. It is usually found at the eastern end of the church, opposite to the entrance.
Now, when we say "altar", we're talking about the table where a priest stands to perform religious ceremonies, like Mass or communion. It's usually made of stone or wood and can be decorated with pretty fancy things like candles or artwork.
The choir stalls are where the singers (called "choir") sit during religious services. They usually sit on benches or chairs that face the altar, but they also can be decorated with pretty designs and carvings.
So, a chevet is basically the fancy area of the church where the really important religious stuff happens!