Light
Light is a call that fighters make when a strike is not believed to be hard enough to count as valid.
The practice comes from the SCA. Fighters are considered to be wearing a maille tunic and a skull cap with a nose piece, regardless of what they are actually wearing. Any shot to the body must hit with enough force to cut through the maille, or any shot to the head must connect with 'positive force.'
An SCA fighter will call 'light' when the shot is believed to be too weak to count. This is not a valid practice in amtgard, and is considered to be Sluffing or Rhino-Hiding.
You may encounter it in some lands however. The rulebook mentions that hits that nick or graze do not count, and this is what is usually interpreted as garb or light shots.
At large Interkingdom events oftentimes you find groups who interpret this differently, quite often it quickly becomes a hardest common denonominator, as people start swinging harder to make sure their shots are called.