Cornerback
The defender responsible for directly covering opposing wide receivers on the outside.
The cornerback usually has the most direct job on defense: run stride for stride with a wide receiver and prevent a successful catch. The position demands top-end speed, quick reflexes, and a good memory for the typical routes opposing receivers run.
The league's best cornerbacks often play "man coverage" — one-on-one, without help — against the opponent's top receiver, freeing the rest of the defense to focus on other threats. That level of trust from coaches is considered a clear sign of a player's quality.
A good cornerback is also measured by how many passes they break up without ever recording an interception — a less flashy stat, but just as valuable.