There are several different personnel groups defenses use, depending on the circumstance. On obvious passing downs, one package teams tend to employ is the nickel package.
There are usually four defensive backs on the field in a base package: two corners and two safeties. In the nickel package, a linebacker, or sometimes a defensive lineman, comes off the field and a fifth defensive backs takes their place. That fifth defensive back is sometimes referred to as a "nickel back."
If an offense is faced with a third-and-long situation, for example, they may bring three, four or potentially five wide receivers onto the field. An extra defensive back helps prevent a situation where a linebacker is forced into a mismatch trying to cover a much faster wide receiver.
The nickel back is usually the third corner on the depth chart. So for the Buccaneers, that player throughout training camp has been Leonard Johnson, while Alterraun Verner and Johnthan Banks worked as the starters.