To sear food is to expose its surface to very high temperatures, typically 400-600°F (200-315°C), so it browns and develops a flavorful crust through the Maillard reaction rather than cooking through.
A proper sear needs a heavy, well-seasoned pan or grill holding a steady high heat, food that is patted dry and at room temperature, and enough room in the pan that the temperature doesn't drop. The food is left undisturbed for several minutes to let the crust form before it is flipped or moved.
Searing is often the first step before braising or roasting: it builds flavor and color on the surface, but on its own it does not cook thick cuts through, and it does not meaningfully "seal in juices" as often claimed - moisture loss depends mainly on final internal temperature and resting, not on the crust. It also works on vegetables such as eggplant, zucchini, and mushrooms.