As the film begins, Caleb is asked to administer something called the Turing Test to Ava, in an effort to see if she qualifies as a true artificial intelligence. If she does, Nathan says, it will be a milestone in the history of man. "If you've created a conscious machine, that's not the history of man," Caleb says. "That's the history of gods."
The Turing test is plenty real -- it's based on a proposal put forth by computer science pioneer Alan Turing all the way back in 1950. The basic idea is, if a computer can engage in natural language conversation in such a way that it's indistinguishable from a human, then the machine is considered intelligent.
In the film, Nathan claims that he needs Caleb to give the test, as an objective outside observer. But since Caleb already knows that Ava is a machine, the entire scenario seems pointless -- the first indication that weirdness is imminent.