A release candidate is something like a beta of an upcoming release. They are pretty well resolved but the community may find some bugs that need to be addressed. Programmers know how their programs work so they rarely do things that would cause a crash, but the average user is much more creative and likely will find things that the programmers miss.
They are put out there for many reasons. In this case ACR 6.7 release candidate appears to be available for compatibility between Lightroom 4 and Photoshop CS5. There are some new cameras supported but there may be more added before the final version. If you have a relatively new camera that is not supported by ACR 6.6 you should check this out. It is not necessary that you download the release candidate as the final version will be sent to you automatically when it is ready for distribution.
The DNG converter 6.7 goes along for the ride. And, of course, the apperance of the release candidate means that the final update is very close. With the release of Lightroom 4, there is a strong indication that ACR 7, which matches the Lightroom version will also soon follow. There will be some major changes in the interface and operation of ACR in the new release with Photoshop CS6.