Complements of <a href = "http://sports.espn.go.com/nfl/columns/story?columnist=pasquarelli_len&id=2352890">Len Pasquarelli</a>
In the simplest terms, cash over cap is essentially the difference between a team's true payroll and the NFL salary cap in a given season. Many of the league's high-revenue teams, but certainly not all of them, have a considerable advantage over the clubs occupying the low-revenue rungs in terms of cash over cap.
To understand the concept of cash over cap, one must understand that the salary cap is just a bookkeeping number, one that can be massaged by amortizing signing bonuses and with other mechanisms. The cap has never been indicative of a team's payroll. The Washington Redskins, believed to be the highest revenue producing machine in the league, have had payrolls well over $100 million the last few seasons, even though the highest salary cap level ever was in 2005, at $85.5 million.
For the fans who can't get their heads around how this works, here's a simple example: Let's say the Redskins signed an unrestricted free agent to a five-year deal that includes a signing bonus of $10 million and a base salary of $1 million for the first season of the contract. In salary cap terms, the Redskins are charged only $3 million, arrived at by prorating the signing bonus over five years and then adding the base salary. But in real dollars expended, or payroll, that player cost the Redskins $11 million for the first year. That's a difference of $8 million between what the player was actually paid and what his cap charge was for the initial season of the contract.
Multiply that example by several player acquisitions, prominent free agents or high-round draft choices, and the total cash over cap is considerable.