It's kind of similar to tax evasion really.
In a way, it's the opposite. Tax evasion is about moving legitimate money
out of of the system such that people can't find it, so you don't have to pay taxes on it.
Whereas "money laundering" is about moving illegitimate money
into the system, such as through a car wash, and making it appear to be legitimate ("clean") money, so you
can pay taxes on it, so that people don't suspect it of being drug proceeds.