The following conversation happens in the movie:
Nick Fury: I don’t know about that, but it is powered by the cube. And I’d like to know how Loki used it to turn two of the sharpest men I know into his personal flying monkeys.
Thor: Monkeys? I do not understand.
Steve Rogers: I do!
(Quotes taken from imdb)
Needless to say – I didn’t understand that reference. What does it reference to – if anything?
Captain America was frozen in ice from around 1943 to around 2012. so he doesn’t know thousands of cultural references like “Here’s Johnny”, Bueller, redpill, etc.
An ongoing joke in the Avengers movie is how those cultural references go right over Cap’s head. He knows they are cultural references, but he doesn’t know what they mean.
This running joke’s denouement is when Cap actually gets one: the Flying Monkeys reference by Nick Fury about Erik Selvig and Hawkeye.
He gets it because it is from the motion picture The Wizard Of Oz, wherein the evil character the Wicked Witch of the West has an army of minions which are literally flying monkeys, which act as relatively mindless (being monkeys) drones.
The Wizard of Oz was released in 1939 and was immensely popular (which is why ‘Cap knows it) and has remained timeless, being often re-released both in theaters and in home media (which is why the rest of the Avengers know it). Decently cultured cities have movie theaters that specialize in classic films and indies, and you can bet somewhere in the world one of them is playing it this weekend. It is that big.