The trading deadline is the last day that major league teams can made trades without a player clearing waivers. Recently, it has become a time for teams who are out of the playoffs to dump expensive players for prospects as they rebuild their team. Also, players who are expected to become free agents are sometimes "rented" to a contender for the pennant race.
The deadline was put in place after the late-season trade of Joe Dugan that allowed the New York Yankees to win the 1922 American League pennant. During the off season, Commissioner Landis enacted a trading deadline.
From 1923 to 1985, the trading deadline was June 15. Since 1986, it has been midnight Eastern time on July 31. Sometimes trades are not announced until August 1 as the paperwork does not reach the league office until after 11 PM on July 31.