Valet is when you go to a fancy restaurant or a hotel and when you get there you drive your car up to a special area where someone who works there (who is called a valet) takes your car and parks it for you.
The person who takes your car is very careful not to damage it and parks it in a safe place. When you are ready to leave, you just have to tell the valet that you need your car and they will bring it back to you.
This way, you don't have to worry about finding a parking spot or walking too far from where you parked to the restaurant or hotel.
It's like having a personal chauffeur for your car!