Chrono mode means that AXti.me RM receives start and stop triggers from your timing hardware, not just a finish time. So a driver crosses the start line, AXti.me RM gets that signal and same with finish line and splits. This allows us to display virtual timers and associate it with drivers in the Queue. Which also means we know who is out on course.
Your timing hardware must support it. Typically FarmTek and RaceAmerica (no-AC4) can be setup for Chrono mode.