Chi Rho is an early Christian symbol, which is still used by some branches of Christianity, such as Catholicism. This symbol is a compound symbol composed of the first two words Chi and Rho of the Greek word “ΧΡΙΣΤΟΣ” (that is, the Greek writing of Christ). That is to say, the mark represents Jesus Christ.

It is known that the earliest symbol of Chi Rho was used to refer to Jesus by Roman Emperor Constantine. He claimed that he got Jesus' Enlightenment from his dream and used the symbol on his flag, which is the so-called labran flag.

Source: @simonhannatattoos