Welcome to the forum
Edit: or, I should say, welcome to
posting on the forum.
An RC is a "release candidate", meaning a build I made that *may* be ready for release (in this case, 0.9.1a), but that will undergo some more testing as a "properly installed, with dev mode off" build etc (rather than me launching the game from the dev environment).
Usually a few more problems will come up, get fixed, and this leads to a new RC being built. So for example if you see the release have a version number like "0.9.1a-RC7" that means there were 6 preceding candidates built that didn't quite make the grade.