Chum Bucket Supreme is simply a bucket of chum. The chum is served in smaller buckets that look similar to the bigger Chum Bucket that also reads "FUM IS CHUM" on them. People eat the chum by putting the bucket around their necks and eating the contents from the pail.
Role in episode
Many of the customers at the Chum Bucket buy a Chum Bucket Supreme only because it has a catchy slogan. They immediately vomit it out into the bucket after eating it.