A bib is the numbered tag runners pin to their shirt to identify them during a race. It usually carries the timing chip, and it's what officials, photographers, and aid station volunteers use to recognize each runner.
It identifies you during the race and usually carries the timing chip that records your result. Some bibs also print emergency contact or medical information on the back.
Almost always at bib or packet pickup, typically held at a race expo in the day or two before the event. Bring your registration confirmation and a valid photo ID.
Yes, it's required for timing, entry into your starting corral, and access to aid stations. It should be visible on the front of your shirt, not tucked away.
Contact the race organizers or help desk immediately. A replacement may be issued, though it can affect how your timing is recorded.
Most modern races embed the chip directly into the bib rather than using a separate ankle strap. That's why you shouldn't fold or damage your bib before race day.