The only guaranteed money in an NFL contract is the signing bonus. The rest of the money is dependent on the player's stauts with the team.
For example, say a team signs player X to a 5-year contract at escalating pay of $550,000 the first year, $750,000 the second, $1 million the third, $1.2 million the fourth and $1.5 million the fifth year. That's $5 million. In addition, they give him a $2.5 million signing bonus. Total value of the contract is (a highly simplified) 5-year, $7.5 million contract. At first blush, this looks like a $1.5 million per year deal.
Player X blows out a knee on his way to mini camp and never plays. The team is still out the $2.5 million and that will count against the salary cap. However, the team doesn't pay player X another dime. Or the team decides the guy's a stiff so they cut him after the first season. Because he's not on the team, he doesn't get the pay for years 2-5. But he still keeps his signing bonus.
Harsh reality. That's why players fight so hard for the signing bonus. It's their only guaranteed money in an uncertain business. Uncertain from the players' perspective, that is.