Scrims are pieces of screen that are either green which takes the light down a half stop. Or, they are red which takes the light down one stop.  You can have half scrims as well that only work with Fresnel lights.  I use the half scrims to balance the light so that it delivers an even field across an area. 

Say, I was lighting a back wall and wanted the light to be even at the source and at the other end of the wall. I could use the half-scrim.  Half singles work very well this way.  I also use them in condors to put the half scrims at the bottom because this will always be brighter.