On the blockchain, accounts have a certain amount of currency.

You can issue a command to transfer currency from your account to somebody else's, as that is a primary use case of a cryptocurrency. There was a code path where you could send someone negative amounts of the currency and it would happily pay them a negative amount of currency and charge you a negative amount of currency, thus transferring their account balance to your against their will.

There were several transfer paths and I think not all of them were vulnerable, but only one has to be. There's a bit of indirection that made it somewhat less obvious than my description makes it sound, though it amounts to the same thing in the end.