RC version of a program.

Ref. What is an RC version of a program?

Release Candidate, it means its a candidate for a final (official/finished) release of a program.
programs usually go Alpha>Beta>RC>Final.
alpha should be considered badly buggy.
beta should be considered buggy.
RC should be considered stable but may have some small bugs in it.
final is the final product (but not the last version ever).

Alpha's & Beta's (especialy Alpha's) are usually hidden or kept private to avoid normal people from using them to avoid support/troubleshooting problems since there pre-releases

these are development steps/phases, its for planning when they can release the finished version of that program
you could go thought 5 Alpha's, 3 Beta's & 1 or 2 RC's
its to iron out bugs & make sure the features the developers want are in it & in it stabily.