Plasma is the liquid component of blood. It's produced by taking whole blood and, by using a centrifuge, separating out the red blood cells, white blood cells, and antibodies.
Plasma has advantages over both a whole blood transfusion and using intravenous saline. Unlike whole blood, it does not have to be typed before a transfusion. It also stores for longer periods than whole blood. Unlike saline, it's likely not to displace blood cells or put additional stress on the kidneys.