In college athletics in the US:
When a college athlete takes a year off from playing his or her sport. This is done to preserve a year of that student's NCAA eligibility (which is normally four years.) Usually an athlete is redshirted because they are not likely to play that year due to injury (i.e. they're hurt), grades (i.e. they're dumb) or that there are already better players on the team (i.e. they suck.)