Tornado Cash, a cryptocurrency tumbler, has been sanctioned by the US government. As you can see a sanction like this has some dramatic effects.
To recap, a cryptocurrency tumbler is a service that "launders" your cryptocoin for you. As you know, cryptocurrencies are individually identifiable tokens. Unlike a real money bank account, in which your balance is just a representative number and any bank can exchange your account balance for any random assortment of equivalent-value paper bills, when you trade a crypto coin you're trading that exact coin, identified with a number, and apps are available that can trace a specific coin's transaction history through the various wallets it's been in. Ostensibly, this would make things tough on people who steal cryptocurrency, because you can see what wallet the stolen coins went into and you can see where they go when they leave that wallet, which could help identify the thief.
Here's how the tumbler works. Assume you're a crypto scammer and you've just scored "tens of millions" from some poorly-designed DAO or whatever. You have all the cryptomoney and they can't get it back, but you also can't really spend it because everyone knows those coins are stolen. So, you transfer it - all or some of it - from your hot wallet to the tumbler - a massive pool wallet that dozens or even hundreds of other, unrelated-to-your-heist people are also putting their own money into. Then, at random times, the tumbler sends randomly-sized payments to one or more separate wallets belonging to you but that nobody else knows are yours. Eventually you get all your money back - that is, you get back an equivalent total number of coins to what you put in, but it's split up between several unconnected (as far as anyone else knows) wallets, and while some of the coins you get back might be the known-stolen coins, others will have come from completely different people. The fact that the individual coins have gone through the tumbler can still be seen by the blockchain, but there's no easy way of telling who owns the wallets they went into on the other side. Your stolen crypto has now been laundered!
News reports often describe crypto tumblers as things that can be used for laundering, but the fact is that laundering is exclusively what they do, there is no other purpose for them.