They are grease crumbs that are fried in a deep fryer.
Role in episode
After the Krusty Krab starts selling Krabby Patties with Yummy Bunz, Plankton must once again find a way to outdo Mr. Krabs. He gets the idea to do away with chum and just sell pure grease, which he burns to a crisp and sells as Crispity Crunchities. This also inspires Mr. Krabs to start selling pure grease as well, which he calls "Wow! Soup."
In "Back to the Past," they are referenced by Patrick when he places a quarter in the time machine, thinking it is a vending machine. He claims that he wants his "Crunchity Munchities," although no actual snack machine is shown.
Patrick mistakenly refers to them as "Crunchity Munchities" in this episode.