Click on the Apple icon in the Mac menu bar and choose About This Mac. Click "Version 10.xxx". You should see something like this:
Version 10.10.3 (14D113c)
CFBundleShortVersionString (10.10.3) is the version number and is the version number that is public and marketed to users.
CFBundleVersion (14D113c) is the build number.
Naturally, you should increase the CFBundleVersion (build number) with every build, example for testing - TestFlight, HockeyApp.
You also (and only) increase the CFBundleShortVersionString (version number) when it's a new release, e.g. when Apple starts beta for 10.10.3, all builds should have version=10.10.3 instead of the previous 10.10.2 and all builds will have different build numbers.
Specifically for RubyMotion:
CFBundleVersion = version
CFBundleShortVersionString = short_version || version
So if you don't care about anything, use only version, and at least increment it everytime you have a new build submit for approval, testflight, HockeyApp, etc. RubyMotion will use the same value for both the version number and build number. IIRC, Xcode used to do the same thing until roughly 2014 or so where you had to explicitly specify both.
If you want to be fine and proper, update short_version with every build and increment version when you want to begin testing a new version.