Here's how it works: If you've reached level 100 in Payday 2, you can pay $200,000,000 from your in-game offshore bank account to essentially trade in your rank. You'll go back to level one, lose all of your spending money, level progress and skill points, but you'll gain an Infamy level – which yields a point you can spend in the new Infamy skill tree. There are 14 levels of Infamy in all, each split into four tiers.
You won't lose all your guns, nor will you lose your mods and masks, but as a lowly level one criminal, you'll be forced to climb the criminal ladder once more to unlock the ability to wield all your good gear. In layman's terms, Infamy is essentially Payday 2's version of the popular Prestige system seen in the Call of Duty series.
The Infamy DLC is available right now as a free update, which will trigger the next time you boot up the game.