Reverse sear just reverses the order of cooking.

A "normal" sear is you putting the raw steak on the pan, and that frying is what actually cooks the steak, from the outside in.

A reverse sear cooks the steak first, getting the whole thing to (or close to) temp, and then very quickly searing the outside at very high heat - the speed means the inside doesn't cook much more, but you still get the delicious crispy bits on the outside. It tends to give you a more even cook inside and a better sear outside - the only drawback is that there are more steps and it takes longer 🤷‍♂️