According to tradition Elijah will come to."pave the way" and announce the Mashiach's arrival. He is intrinsically connected to the final redemption and salvation of the Jewish People from all enemies and problematic circumstances in this world.
Since Pesach was a time of freedom and redemption from Egypt - and according to our Sages the future redemption will occur in the month of Pesach which is Nisan - we symbolically invite Elijah to our celebration of our past redemption by opening the door for him and setting a special cup of "wine of redemption" for him at the Seder, as we read in the Hagaddah. With great hope and prayer for the future redemption, may it be very soon.