Target share measures how often a receiver is involved when his team throws the ball. If a team throws 40 passes in a game and a wide receiver is targeted on 12 of them, his target share is 30%. The number is usually shown as a percentage and can be tracked for a single game, a full season, or several seasons.
In fantasy football, target share is one of the most useful stats for predicting future scoring. Wide receivers and tight ends who earn a large share of their team's targets get more chances to catch passes, gain yards, and score touchdowns. A high target share suggests the coaching staff trusts that player and that his production is likely to continue, even if he has had some unlucky games.
Target share is especially important in PPR leagues, where each catch adds a point. A player who catches a lot of passes because of a big role in the offense can be more valuable than a player with similar yardage but far fewer opportunities.