This is part of how the notion of security is defined. In the definition, the adversary is an algorithm that produces a pair of messages, and then gets back (in return) a ciphertext. The adversary is then supposed to do something (predict whether the ciphertext is the encryption of the first message or the second message), but that's not relevant -- what is relevant is that this algorithm receives a ciphertext from its oracle immediately after selecting a pair of messages.
If you like, you can think of this as a game, with a referee, an adversary, and an oracle. The adversary selects a pair of messages somehow and sends them to the oracle. The oracle responds by sending a ciphertext back to the adversary. The adversary receives this ciphertext, does some computation, and then outputs a bit. We define the condition under which the adversary wins, and under which the adversary loses. I won't go into the details of that condition; for the purposes of your question, all I need point out is that this game involves the adversary receiving a ciphertext from its oracle at a certain point in the game. That's the "receive" part you were asking about.
This should be explained in any textbook or good course on theoretical cryptography.