They are essentially robotic shells that hold organic consciousness, originally meant as a means to remove vulnerable organic forms to combat the Flood virus.
The Didact "digitized" ancient humans with the Composer and transferred their minds into Knight forms.
This leads to the plot of Halo 4; the Didact is released and he intended to use the Composer on the people of Earth to create another Promethean army. --- How many lasers does it take to open a bank account at McDonalds?
It just seems odd that he needed to sacrifice Prometheans and Humans to make robots. The Forerunners were already capable of making robots.
Perhaps just upload a single Promethean consciousness to a forerunner super computer, right click on the file, click on copy, go to desktop, right click paste. Just copy after copy, ready to be installed on robot units.
Another question, are other forerunners alive aside from Didact? His speech at the end sounds like he's speaking to perhaps a group of forerunners.