A potluck is a usually the name of a meal where everyone brings something, usually unplanned. Or, it may mean a meal that is made from whatever is available for an unexpected visitor. It may be used as an adjective or noun. When used as a noun, it is made plural by adding an s.
It comes from the sixteenth century where an author used pot-luck to mean the unexpected guest got the “luck of the pot” or whatever was already prepared.
There is also an Irish tradition of everyone bringing available ingredients and throwing them all into one pot for dinner.