The Avengers premiered on April 11th, 2012 in the United States.
In one of the scenes, Nick Fury (portrayed by Samuel L. Jackson) is talking about Loki having control about two of his agents, calling them Loki’s “personal flying monkeys”, making a reference to The Wizard of Oz.
The only one who gets this reference among the Avengers is Captain America, who exclaims “I understood that reference”.
The earliest post involving the quote was uploaded to Reddit’s r/funny subreddit by GeneralBacon on May 22nd, 2012.
The quote would then grow to become a reaction meme, with the first GIF having been uploaded to Imgur on December 18th, 2012.
“I understood that reference” would be a common reaction image for expressing and understanding for a reference on sites like Reddit, Imgur, Tumblr or Pinterest.