The swear jar is a plastic container used in the collection of 'restitution' in the form of US currency (dollars) for the usage of standard American swear words and phrases during the show. The fee is $1USD per word or phrase.
It is not known what the money that is collected is used for.
The swear jar was first used in episode 349.
Yappy Fox brings down the Swear Jar on Poink during episode 349.