Following a series of beta releases, final testing begins with RC1
This is the time when developers, hosts and enthusiast who want a sneak peek into how their sites with the latest release, but don’t want to take part in the core development cycle, really really should test.
Release candidates are like being given a pre-release. It is stable. It has been tested. Unless YOU find any issues, this will be the next release of WordPress. So get stuck in!