Short answers, pulled from the story.
The baseball color line was a racial barrier, sometimes called a "gentlemen's agreement," that excluded Black players from Major League Baseball and its affiliated minor leagues. It had no written rule at the major league level but was enforced through collective understanding from the late 1880s until 1947, when Jackie Robinson joined the Brooklyn Dodgers and Larry Doby joined the Cleveland Indians.
Moses Fleetwood Walker was the first recognized Black player in Major League Baseball, debuting as a catcher for the Toledo Blue Stockings of the American Association on the 1st of May 1884. His brother Weldy Walker also played for Toledo during part of that season. No other known Black player appeared in the major leagues for around 75 years after the Walker brothers.
Cap Anson, star player of the National League's Chicago franchise, repeatedly pressured teams not to play against Black players in exhibition games, making demands in 1883, 1884, and 1887. His 1887 telegram to the Newark Little Giants demanding they bench Fleet Walker and George Stovey came on the same afternoon the International League voted to bar new contracts with Black players. Black player and author Sol White wrote in 1907 that "Were it not for this same man Anson, there would have been a colored player in the National League in 1887."
Branch Rickey, general manager of the Brooklyn Dodgers, signed Jackie Robinson in October with the support of new commissioner Happy Chandler. Robinson played the 1946 season for the Montreal Royals and joined the Dodgers in 1947, winning the first Rookie of the Year award ever given, which now bears his name. Larry Doby simultaneously became the American League's first Black player that same season, signed by Bill Veeck for the Cleveland Indians.
On the 14th of July 1887, the International League voted 6-4 to stop allowing new contracts with Black players, with all-white teams voting in favor and clubs with Black players voting against. Secretary C.D. White was instructed to approve no further contracts with Black players. Only two players remained in the league for 1888, Frank Grant and Moses Fleetwood Walker, retained solely because their contracts predated the vote.
The Boston Red Sox were the last major league team to integrate because of the persistent resistance of owner Tom Yawkey. Despite giving Jackie Robinson a brief tryout at Fenway Park in April 1945, the team refused to sign him. The Red Sox did not integrate until the 21st of July 1959, when Pumpsie Green debuted as a pinch runner, following a lawsuit and charges of discrimination from the NAACP and the Massachusetts Commission Against Discrimination.