Salsa Imbecilicus resembles regular Chum sauce, but added with Patrick's DNA. It is known to have a foul stench.
Plankton creates Salsa Imbecilicus after getting the idea from Sandy to make Mr. Krabs be as dumb as Patrick. After getting the DNA from Patrick, Plankton finishes making his salsa. Unfortunately, when he is mixing up the salsa, Plankton gets some on himself and it turns him dumb. The fumes from Salsa Imbecilicus end up escaping through the vents of the Chum Bucket and into a cloud that eventually gets into Bikini Bottom and turns everyone dumb (except for Sandy, who is safe inside her treedome, and Karen, who is a robot).
A lot of people were confused by the word "imbecilicus" in the episode title, until they found it was a made-up word used by Plankton for the name of the sauce he created.