Mr. Krabs' mattress is a mattress that belongs to Mr. Krabs. It is where he sleeps and keeps his money. In the episode "The Lost Mattress," it is replaced with another mattress by SpongeBob, Patrick, and Squidward. The mattress is thrown away in the dump and used by a guard worm, but Krabs gets it back when he reaches said dump.
Mr. Krabs kept his money there and slept on it for a long time. Years later, his back is not in a good state because his mattress is "lumpy." When SpongeBob finds out, he finds a new mattress for him and replaces the old mattress.
When Mr. Krabs tries it out, he falls fast asleep, but when he finds out that Squidward threw his mattress in the dump, he is not pleased. He faints and goes into a "cash coma," so there is no choice but to retrieve it. SpongeBob, Squidward, and Patrick try to get the mattress back, but are unable to because of the Worm.