In basketball, a turnover occurs when a team gives up possession of the ball before they have made an attempt at a shot. The turnover can occur because the ball is stolen, because someone attempts a basket that does not hit the rim or backboard, because the offensive team commits a foul that makes them give up the ball, or because of a violation of the shot clock.
For obvious reasons, turning over the ball is a negative thing to do, so statistics on turnovers are often looked at to see why a team is losing a game, or why a player might not be as good as their positive statistics reveal. However, like most statistics, turnovers can be deceptive. After all, the only real statistic that matters in basketball is the Win/Loss column. To show why turnovers can be deceptive, imagine a player who has the ball when the shot clock is about to run out, and who does not have a good shot available. They can make the shot, and miss; or they can not shoot and commit a turnover. In either case, they don't score, but in the second scenario, they earn a turnover. For this reason, although turnovers are not good, they don't tell the entire story, because there are many reasons an offense fails, and turning over the ball is just one of them.