A red scrim is a "double", a green is a "single". A red scrim will cut light about one full stop so its not as harsh. They are named such for the color around the ring of the scrim.
1/2 blue refers to CTB (color temp blue) gel, which is used to correct the orange light from tungstens to more closely match daylight, which is blue. They come in all strengths, from 1/4CTB, 1/2, full, etc. As you can imagine, there is also CTO, to correct blue to orange. For more on this, look into color temperatures and degrees kelvin. Wikipedia and google are your friends.
