Losses measure the total number of time a pitcher is awarded the loss in games over the season. Pitchers are awarded the loss when they allow the go-ahead run to reach the base and that pitchers team never regains the lead or gets to a tie again in the game.
More about Losses Leaders
Those who lead the league in pitching losses must walk a fine line. They must pitch poorly enough in many games to be credited with the loss, but they must also pitch well enough in order to keep being given opportunities to start and have more chances to earn another loss. Leaders in losses are most likely to pitch for poor teams, for multiple reasons: 1) the team’s offense must be poor enough to consistently not score enough runs, and 2) the team is unlikely to look for other options if a pitcher is consistently pitching poorly (i.e. a team with a chance to make the playoffs has more motivation to look for alternatives, whereas a team likely to miss the playoffs isn’t likely to spend additional money/resources on other pitchers).