oUF is a custom unit frame framework built upon the World of Warcraft API. Its goal is to release developers from the pain of updating the frame, and let them so to say freely focus on the interface itself.
Currently the a.b.c scheme is used for versioning. Out of these three values only b and c are important. b is used to represent the current API compatibility. While c is used to represent current maintenance release on that API branch. The basic idea is that API breakage isn’t allowed without increasing the API compatibility number to reflect this. A example would be that a layout released for oUF 1.3.1 should work when oUF has been updated to 1.3.28. In other words, breaking changes made without a API bump is considered a bug in oUF.