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 (master)

HEAD, also known as -HEAD or “master”, refers to the development version of pfSense software, 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.

RELENG_2_3

This branch is under active development for the 2.3.1 release.

RELENG_2_3_0

This branch is under active maintenance for 2.3 errata.

Others

For links to other branches, past and present, see Versions of pfSense and FreeBSD