A money mule is a person who accepts money or goods and then forwards the aforementioned to a 3rd party.
The money or goods will have been acquired though deception, either monies that have been acquired via phishing or goods purchased via a stolen credit card, for instance.
The mule will take a commission or be paid a percentage of the goods value in return for laundering the transaction and sending money or goods to the destination country.
Many people become money mules under the pretence that they are in gainful employment & so become unwitting participants in a crime (Proceeds of Crime Act of 2002, UK).