The fifth cup of wine at seder, for Eliyahu (Elijah) the prophet, symbolizes v'heiveiti--“I will bring them.”
Reason In the future, everyone will drink five cups of seder wine. (Some say the term v'heiveiti is not part of the redemption and some say it is a stage that has not happened yet.)
The cup of Eliyahu (which is not intended to be drunk by Eliyahu) should be used for kiddush the next morning. Various customs dictate when to fill Eliyahu's cup; it may be filled anytime from the beginning of the seder.