Every season on September 1st, Major League Baseball teams are allowed to expand their 25 man active rosters. Each club has the ability to bring up to 15 additional players to the big leagues with a full 40 man roster. Teams almost never call up that many players, of course, but this is a time of year when contenders may call up a couple of power bench bats, additional relief help, or base stealing specialists that may give them an edge in September.
Call ups often come in waves, with perhaps a couple of players coming right away on the first of September and then more reinforcements once the minor league season (and playoffs, if a team qualifies) is officially in the books.