What is an Erlang hrl file?

The actual doc is http://www.erlang.org/doc/reference_manual/macros.html

A "header file" akin to a "C" include file.

One needs to invoke the -include directive from an .erl file to use them. Can contain any valid Erlang code.

hrl files are simply erlang "header", that is files containing common definitions that are intended to be included by .erl files.