system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 please, the different defensive formations in American football. I'm listening to sports talk radio, and they just mentioned that this one guy would be used in nickel packages. I'm a football fan, but I just don't understand what these mean.
gpt
A nickel defense is used when the defense expects the offense to pass--when the team on offense has to get a lot of yardage, or perhaps is behind and needs to score quickly. 

A nickel defense uses five (hence the name) defensive backs--that is, players who play well back to cover receivers from the offense coming down the field to catch passes. Normally, a team playing on defense would have only four players back.

Teams could also run a dime defense, which adds an additional defensive back to the nickel to make six players defending against the pass.