In short: a money mule is a person whose account is used to pass on other people's money from fraud — they receive it and forward or withdraw it for a small reward, and so take part in money laundering.
Simply put: The offer sounds harmless: "I'll send you money, withdraw it and give it to me, €50 is yours", or "work from home as a financial agent". Young people and students are often targeted. Europol warns that this is a crime even if the person did not know where the money came from.
Example: A stranger on social media offers Sasho €50 to receive €800 into his account and send it on. Sasho refuses and tells his parents.
Watch out: never give your account, card or banking access to anyone else — not "just for a moment", not even to a friend.
What to do: if you have already received such money — stop forwarding it, call your bank at once and report it to the police. The consequences depend on each country's law.
Source: Europol — "Money muling" leaflet (#DontBeaMule campaign): https://www.europol.europa.eu/sites/default/files/documents/lux_flyers_en.pdf