In theory, it’s such a simple concept. In practice, it’s been impossible for any player to get it.
To protect a player who signs a long-term deal against the contract becoming obsolete, the player gets a guaranteed percentage of the salary cap in every year of the contract.
Multiple teams have tried to get that term. A decade ago, former Jets cornerback Darrelle Revis tried to get that term. The team refused. Quarterback Kirk Cousins, while under the franchise tag in Washington, tried to get that term. The team refused.