What is meant by calling the lights in live concerts or theater?

In a live performance, the lighting cues have to fire at precise times, and those times have to be called by one person. The person who calls lights at a theatrical performance is the stage manager, and at a concert, it is usually the lighting director.

One of these two people is responsible for staying on the headset and calling for cues to be executed to the light board operator and to the spotlight operator. This is known officially as "calling the light cues", but for short we call it "calling the lights".