I've heard it both ways. First-to-file either means:

- A patent cannot be challenged on the basis of prior art once issued

or:

- If two people independently invent something, and both apply for the patent, the first one to get to the patent office gets the patent, rather than the first one to document the idea. That documentation would still be prior art, though.

People who think it's the latter tend to like first-to-file. People who think it's the former tend to not. I haven't actually heard from a patent lawyer about it, though.