the green lights which? the straight line at the start of the runway? if so they are for the threshold. the red lights at the end of the runway are a warning for a certain amount of length left ie, if your on your take off roll, get in the air or your overrunning, and if your landing better add extra brakes.
corrections are always welcome as it was along time ago that i last read about them

Also there are four lights off to the left side (as you are on final) that can be either red or white.  These lights are used to show if the aircraft is high or low.  Idealy you want to have two red and two white all the way down.  If there is more red, that means that your approach is low.  If there is more white it means that the approach is high.