A college football player who is an academic sophomore (2nd year of college classes) but in his first season athletically. They are allowed to practice with the team, but do not get to play in a sanctioned game in their first year at the school. This is different than a true freshman, who is in his first year out of high school.
Reasons for being redshirted are than there are (better) upperclassmen playing the same position, or the player is injured and not able to play the first year. Redshirts get all benefits of being part of the team but don't use a year of athletic eligibility.