What are HEAD, RELENG 2 1, RELENG 2 0, etc
This is the terminology used to refer to a specific git branch of pfSense source code.
HEAD, also known as -HEAD or "master", refers to the development version of pfSense, where all new features are first added. When a release nears, HEAD is branched to a RELENG (Release Engineering) branch. This follows the FreeBSD project's development model.
This branch is under active development for the 2.3.1 release.
This branch is under active maintenance for 2.3 errata.
For links to other branches, past and present, see Versions of pfSense and FreeBSD