(idiomatic, by extension) A prelude or introduction to an activity or thing.
Example: 1910, Edith Wharton, "The Bolted Door" in Tales of Men and Ghosts:
[H]e had tried all kinds: comedy, tragedy, prose and verse, the light curtain-raiser, the short sharp drama, the bourgeoisrealistic and the lyrical-romantic.
2014 Feb. 20, Brian Seibert, "Dance: Next Step for Force in Ballet," New York Times (retrieved 24 Aug 2014):
â€œCapricious Maneuvers,â€ the eight-minute romp of a curtain raiser that Mr. Peck made for City Balletâ€™s 2013 Fall Gala, was fun.
1901, Harold MacGrath, The Puppet Crown, ch. 7:
It is my opinion that your part in the affair is only a curtain-raiser to graver things.
2008 Sept. 11, Virginia Lopez, "Russia's Venezuela Foray: Tit for Tat," Time (retrieved 24 Aug 2014):
But the first foray by Russian strategic bombers into the Western hemisphere since the Cold War is simply the curtain raiser for joint naval maneuvers that will bring Russian warships into Venezuelan waters in November.