In the sport of tennis you often hear the term "open era". How exactly were Grand Slam tournaments different before that?

https://en.wikipedia.org/wiki/History_of_tennis#Open_Era

Prior to 1968, the GS tournaments were strictly amateur only, professionals couldn't take part. From 1968, they became "open" to everybody.

Because the professional men (there was no women's pro tennis tour) couldn't play in the designated grand slam tournaments prior to the Open Era, there were professional equivalents set up for three of them: the U.S. Pro (U.S. Open), the Wembley Championships (Wimbledon), and the French Pro (French Open). (Because it was so expensive to travel to Australia, there was no pro equivalent -- and even the regular Australian Open tournament went through a slump in popularity before worldwide TV broadcasts enabled it to bring its prize money up (in the late 1980s) so that it became worthwhile for the pros to travel there. John McEnroe, for example, played in Australia exactly 5 times, and the first wasn't until 1983, by which time McEnroe was already a four-time Grand Slam winner.) Exactly two players won the "Pro Slam" (all three in the same year): Ken Rosewall in 1963 and Rod Laver in 1967. [Laver then won the only Open Era Grand Slam among the men in 1969 -- although Novak Djokovic is still alive for the Grand Slam in 2021.]

https://en.wikipedia.org/wiki/Major_professional_tennis_tournaments_before_the_Open_Era