That is - if you research a technology with 20 steps, you currently done 6 units, and you have 3 labs currently working on it, the bar would be 6/20 parts green, 3/20 parts light-green, and 11/20 parts black.
The idea is to help you see how fast your research is going, plus allow you to see if you have a problem in the research (if you have no light-green part it means your labs stopped working for some reason).
Just for a visualization - here is how it looks in Civilization 4:
![Image](http://img508.imageshack.us/img508/9168/civ4screenshot0039rn0.jpg)
(For those who don't know civ4 - the top green bar shows research. the first light-green part is how much you currently have researched, and the darker semi-transparent part to the right is how much research will be done in the next turn)