Counterfeit is when something is forged to imitate something else that has a legal validity to it in some way, usually of a financial value, for the purpose of collecting the value of the imitation as though it were the original.
Counterfeit happens with any type of crime in which a document is unlawfully created to imitate a valid document. It is most often related to money laundering and creating money to pass off as valid money.
The crime of counterfeiting is a felony offense and usually carries another related fine, uttering and publishing, which is the act of taking the counterfeit document and trying to pass it off as a valid document. This could be taking counterfeit money to the store to buy something, or trying to file a will that has been forged, or any type of written falsification of something that would otherwise hold a set legal validity to it.