Simon of Cyrene is a peripheral figure in Christianity, who is mentioned in Mark 15:21 and its parallels in the other two Synoptic Gospels (Matthew 27:32 and Luke 23:26). In those passages, from the Passion narrative, Simon is a bystander who is called upon by the Roman guards to carry the cross of Jesus.
Mark:"A certain man from Cyrene, Simon, the father of Alexander and Rufus, was passing by on his way in from the country, and they forced him to carry the cross." 
Matthew: "As they were going out, they met a man from Cyrene, named Simon, and they forced him to carry the cross." 
Luke:"As they led him away, they seized Simon from Cyrene, who was on his way in from the country, and put the cross on him and made him carry it behind Jesus." 
In John's Gospel, however, it is explicity stated that Jesus did not receive any assistance:
"Carrying his own cross, he went out to the place of the Skull (which in Aramaic is called Golgotha)." 
The reason why Simon was called upon is not mentioned, but it is commonly assumed to have been the case that Jesus, weakened from his torture at the hands of the Romans, was unable to carry his own cross. A more intriguing possibility is that this portion of the narrative is a surviving fragment from a different account, in which another man took Jesus' place on the cross. This version of events is still believed in Islam and Gnosticism.