Signing bonus money already paid to the player. Unlike the NHL, in the NFL teams take on cap hits based on what they actually pay the player.
Brady for example, cost the Patriots like $17 mill in dead cap last season.
Brees will cost the Saints over $20 million, but he re-did his contract so it lessens the blow next season, but really just splits it over 2 seasons.
For guaranteed money, there are 2 main ways to give that to a player. Either via signing bonus, or you guarantee base salary early on during the contract. SF did that with Jimmy G, with only like a $7 million SB. His salary for the first couple of seasons was fully guaranteed, thus since they had cap space in those years, they used it rather than spread a signing bonus over multiple years.
Other teams who are up against the salary cap use a SB to push a cap hit into the future, with a rising salary cap.