A tally light is a visual signal used in video production to show when a camera is live (being broadcast or recorded). It tells the presenter which camera to look at and tells camera operators which camera is currently on-air.
Camera is on-air. You're being broadcast or recorded.
Camera is next up. Get ready to go live.
Heads up — something is about to change.
Camera is not active. Safe to move or adjust.
Traditional tally lights are dedicated hardware units ($200–$600 each) that mount on cameras and receive signals from a video switcher over a local network. Modern browser-based tally lights like Cue Light use any smartphone as a display — free, no hardware required, works over the internet.