So before the film school nerds explode, let me just explain what a ‘scrim’ actually is and what the term colloquially means to us photographers.
Technically scrims are large sheets of black woven fabric that reduce the brightness of a light when placed in front of them (these are usually constant HMI lights that don’t have individual brightness adjusters).
There are alternative large sheets of fabric that are placed in front of lights called ‘silks’ and these are actually white in colour. When ‘silks’ are placed in front of the lights, they diffuse the light and spread it over a far wider area resulting in very soft lighting. You could consider that a softbox is actually just a small silk as it has a large sheet of diffusion material in front of the light that softens it as it passes through it.