What does Mark 15:21 mean?

John 19:17 says that Jesus carries His own cross. Despite popular depictions, Romans typically left the vertical part of crosses fixed to the ground, since it would have been overly difficult to take them up and down. The railroad-tie-sized horizontal beam was often carried separately to the site, by the prisoner themselves. The traditional interpretation is that Jesus starts carrying the cross-beam His hands will be nailed to, but the beatings have so weakened Him He cannot continue.

There is conflicting information as to where Jesus starts out; it may be the Palace of Herod, or the Fortress Antonia on the northwest corner of the temple mount. Neither do we know exactly where Golgotha is, other than outside the old city walls of Jerusalem, possibly on the site of the Church of the Holy Sepulchre. The church is about 2,000 feet, or 600 meters, west and a little bit south of Fortress Antonia.

"Compelled" is from the Greek root word aggareuo. It is the same word used in Matthew 5:41: "And if anyone forces you to go one mile, go with him two miles." A Roman soldier has the authority to press a bystander into service for one mile. As Simon is coming into Jerusalem, he's forced to turn back and carry the cross-bar outside the city walls.

Cyrene is a Greek colony in modern-day Libya. It's entirely possible that Simon is a dark-skinned man of African descent, but Cyrene is also home to a large Jewish community and "Simon" is a Jewish name. Mark does not often give names of those Jesus associates with fleetingly. It's likely his readers have knowledge of at least Alexander and Rufus, if not Simon, himself. It's possible Rufus is the same Rufus mentioned by Paul in Romans 16:13.