Original: The meet-in-the-middle attack uses a space-time tradeoff to find the cryptographic key in only encryptions (and space) versus the expected encryptions (but only space) of the naive attack.
Simplified: The meet-in-the-middle attack attack uses a space-time tradeoff to find the cryptographic key in only encryptions (and space) compared to the expected encryptions (but only space) of the normal attack.