The Postage board is a controller board meant to aid people in making a hand wired keyboard. Controller boards are essentially the brain of the keyboard. It is similar to the common Teensy 2.0, and also the Pro Micro.
However, neither the Teensy, nor the Pro Micro are designed for keyboards specifically. They are just general Arduino-like boards meant for a variety of DIY projects. The Postage Board is designed for mechanical keyboards specifically, and thus the process of assembling your keyboard is easier, and the final product is more refined.
Make example for this keyboard (after setting up your build environment):
make handwired/postageboard/r1:default
or
make handwired/postageboard/mini:default
See the build environment setup and the make instructions for more information. Brand new to QMK? Start with our Complete Newbs Guide.