Communion is a memorial in which Christians identify with the crucifixion of Jesus Christ and remember the Lord's broken body and the blood he shed for us. Along with baptism, Communion is a Covenant sign.
1 Cor 11:25 (ESV) In the same way also he took the cup, after supper, saying, “This cup is the new covenant in my blood. Do this, as often as you drink it, in remembrance of me.” [cf. Matt 26:27-28]