At the start of the battle round, you can select one of the Plagues listed in Nurgle's Gift. Until the end of the battle, that is your chosen Plague instead of any previously chosen Plague.
To see why that matters, remember what the army rule normally does. You pick your Plague during Declare Battle Formations, before the first battle round, and you are stuck with it for the whole game. It is one of the few genuinely irreversible decisions in a Death Guard list, and it is made before you know how the game will go.
This detachment converts that into a decision you make up to five times. Rattlejoint Ague when your opponent's saves are the problem. Skullsquirm Blight when a melee army is about to swing, or when a shooting army is about to fire and you would rather they had cover working against them. Scabrous Soulrot in the round the mission is decided by Objective Control.
Read the wording once more, though, because it is a swap rather than a stack: the new Plague replaces the old one, it does not add to it. Two enhancements in this detachment, Final Ingredient and Cornucophagus, are the ones that add a second Plague on top, and both are narrow.
At 2 DP it leaves 1 DP spare at a 2,000-point game.