The Chi Rho, Christogram is a monogram constructed from the first two letters of Christ in Greek (CRistos/Christos). See the reverse of the coin above for an example.
In 312 A.D., Constantine dreamed he saw a Chi Rho Christogram in the sky and heard the words IN HOC SIGNO ERIS, meaning "In this sign you will be the victor." He ordered the sign of Christ on his legions standards and shields. He won a great victory and later became the first Christian Roman Emperor. He also made Christianity the official religion of the empire.
The Chi Rho symbol is frequently found on the reverse of coins issued after Constantine's victory, as a control mark, and on labarum (Chi Rho Christogram standardards) held by the emperor in various poses. There is a variation of Chi Rho with a vertical cross, see the coin of Arcadius above (lower right).
Magnentius issued the coins with the magnificent chi-rho reverse to curry favor with orthodox Christians who were disgruntled with the Arianism of Constantius II. Arianism maintained Christ was a created being, and that there was a time when he "was not". The orthodox viewpoint is that Christ is part of the eternally existent Trinity, so there was never a time when he did not exist. The KEY to the Magnentian reverse, however, is not the Chi-Rho, but the A (alpha) and the W (omega) to either side. This is a direct allusion to the Revelation of John 1:8 (also 22:13) where Christ says: "I am the Alpha and the Omega, the beginning and the ending," saith the Lord, "which is, and which was, and which is to come, the Almighty." This clearly asserts the eternal existence of Christ, which the Arian Constantius would not support. The west tended to be more orthodox, while the east tended to be more Arian. Magnentius' revolt was of course centered in the west, in Gaul. The presence of the A and W clearly makes it strictly an appeal to orthodox Christians, nothing to do with any meaning the pagans might assign to it.