The minimum gap to the rider ahead on the bike leg of a non-drafting race. Under World Triathlon rules it is 12 metres, and an overtake must be completed within 25 seconds.
The box is why your race looks nothing like the one on television: elite World Triathlon Championship Series racing is draft-legal and turns into a road race with a peloton, while your age-group start is almost certainly non-draft. The usual offence isn't deliberate, it's guesswork — 12 metres is shorter than most people assume, so pace the gap out once in training instead of feeling it for the first time on race day. Officials enforce breaches with time penalties. Two things that get missed: the number isn't universal. World Triathlon works with a 12-metre box, front wheel to front wheel, and a 25-second window to complete an overtake; US federation rules use roughly 7 metres. Whatever applies in your race is in the athlete handbook — look it up rather than assume it. And the ban covers the bike only: in the swim, drafting is expressly allowed and saves energy.