It overwrites all data with 0s. That's it. The data is gone for all intents and purposes. The drive certainly can't read it again...
it's technically possible that the old data still "shines through" - as in where there used to be a 1, there's now a 0.0001, and where there used to be a 0, there's now a 0.
However unless you happen to have lost the meaning of life, or the location of Osama Bin Laden on that drive, you're unlikely to get half the scientists in the world working on recovering it.
this is called "magnetic remnance", the other concept is "track overlay", which is where the heads don't write to exactly the same spot each time, so there are traces of previous data at the edges.