It was a common means of killing prisoners of war in the Ancient Near East and Ancient Far East. The crosses would be hung by roadsides and bodies left indefinitely in order to scare folks into not opposing the empires that had control of the regions.
Simply put, the meaning of the cross is death. In ancient times (i.e., from about the 6th century BC until the 4th century AD), the cross was an instrument of death by the most torturous and painful of ways. In Christianity, the cross is the intersection of God's love and His justice. The Romans were not picky in regards to how they would crucify people. The Romans used crosses, poles, stakes, upside-down crosses, x-shaped crosses, walls, roofs, etc. Jesus could have been crucified on any of these objects and it would not have affected the perfection or sufficiency of His sacrifice. Jesus died on something and His death is the sole atoning sacrifice for our sins (1 John 2:2). What matters is that Jesus died for our sins and that His death purchased for us eternal life.