Rest is a cooking term that describes allowing something to sit undisturbed for a few minutes.
Usually it applies to something that has just been cooked. You let roasted meat rest for a few minutes when removed from the oven, or the flame. It lets the proteins relax, making it easier to carve, and it allows the juices to be drawn back into the meat.
You also let bread rest when it comes out of the oven, so that it will be easier to slice, and let cakes rest when they come out of the oven, so that they will be easier to decorate or slice. Pies are sliced more easily when they have rested.
Resting can also be applied to other things, though, such as dough. Some recipes ask you to let a pie crust dough rest, either after mixing it, or rolling it out. This allows the protein in the flour to relax, which will help prevent shrinkage later on during baking.
When you are allowing something that you want to serve hot to rest, cover it with something to retain the heat: tin foil will do, though if you have a silver meat dome, now’s the ideal time to start showing flashing it about.