It depends. If I invented it, and kept it to myself, and don't get around to filing a patent until after you; first to file means you get the patent. I may have rights to continue to use the invention as a prior user, but I'm not familiar with the details there.

If I published it before you filed though, that's prior art. Again, I'm not super familiar with details here of if it had to be published for some time or just the day before is fine.