"He says you're to come to his office at half past eight tonight to do your detention — er — no matter how many party invitations you've received. And he wanted you to know you'll be sorting out rotten flobberworms from good ones, to use in Potions and — and he says there's no need to bring protective gloves."
-- Demelza Robins delivering a message to Harry from Professor Snape (HBP11)
The mucus exuded by a flobberworm is sometimes used to thicken potions (FB). One of Harry’s detentions with Professor Snape entailed sorting out flobberworms for use in Potions class (HBP11)
Used as a thickening agent in potions including Herbicide and Sleeping Draught on Pottermore, and the Wiggenweld Potion on the video games.
"Flob" is British slang for "spit" - refers to flobberworm mucus