A swear jar is a container wherein someone inserts money whenever they use a swear word. It is designed to be a behavior modifier by assigning a penalty for uttering words one does not use in polite company.
A common use of a swear jar is for parents to change their general daily language after a child is born in order to not have their baby's first word be "fuck". It's also used when the kids are in school and influenced by the culture and languages of their friends, some of which the parents don't approve.
On Everything2, the chatbox bot E2D2 has a swear jar feature (which may or may not be active at any one time) wherein the little droid will deduct some cGP and place it in the assigned swear jar for you. You get to pay the penalty if you use one of the words that E2D2 is trained to look for in the catbox. As of the date of writeup, I have 14.89 cGP in my swear jar. I also have 1095.11 cGP so I guess I can go ahead and swear like a sailor.