A corral is a designated starting pen that groups runners by expected pace or bib category before the race begins. It helps large races start in an orderly way, with faster runners nearer the front so slower runners don't create an early bottleneck.
It's a fenced or marked starting zone assigned based on your projected pace or entry category. Runners line up in their corral before the gun fires.
It's usually printed on your bib or listed in your race instructions. Assignment is typically based on the finish time you estimated at registration.
You're generally expected to stay in your assigned corral. Moving up to a faster one is usually discouraged, while moving back to a slower one is often allowed.
They manage congestion right at the start line and reduce the risk of runners with very different paces colliding early in the race. It's mainly a safety and flow measure.
Mostly larger events with thousands of participants. Smaller races often just have a single open start with no formal corral system.