The act of searing is to brown meat quickly by exposing it to very high heat in a frying pan/skillet, or under a broiler. This will seal in the meat’s juices by generating a caramelized outside crust.
When practicing how to sear a steak to the optimum internal temperature use an instant read meat thermometer. See below for basic temperatures for different degrees of doneness and remember to account for carry-over cooking and resting which will continue to cook the steak.