In computing, firmware is a computer program that is embedded in a hardware device, for example a microcontroller like the Make Controller.  As its name suggests, firmware is somewhere between hardware and software. 

Like software, firmware consists of code to be run by a computer.  But, firmware usually implies that the program is only designed to run a particular piece of hardware. 

Imagine, for example, all the different kinds of computers that Windows, OS X, and Linux need to be able to run on.  The programs that run on these machines are examples of software, whereas the Make Controller is a specific piece of hardware with its own programming interface.

The default firmware for the Make Controller Kit is (arbitrarily) called Heavy.

The firmware API for the Make Controller Kit can be found at http://www.makingthings.com/ref/firmware/html