What is a false start in track?

A false start is leaving the blocks before the gun, or reacting faster than 0.100 seconds after it. Under World Athletics rules the athlete responsible is disqualified on the first false start.

Why 0.100

At major meets the blocks carry sensors that time each sprinter's push against the gun. The rules hold that nobody hears the gun and moves within 0.100 seconds, so a reaction faster than that counts as anticipating it. A fast, legal reaction is usually between about 0.120 and 0.180.

One and done

Since 2010, World Athletics disqualifies any athlete responsible for a false start, with no warning first. In combined events the rules allow one false start per race; the athlete responsible for any later false start in that race is disqualified. School and club meets may use their own rules, so check the meet's.