The chi rho was a Greek shorthand symbol created from the letters chi (X, pronounced /ks/) and rho (P, pronounced /r/). It was used as shorthand for the words Khristós (Christ) and khrēston (good).
The chi rho is best known as a symbol representing Christ, and is one of the more common Christograms, a stylized monogram of Jesus Christ's name. Chi rho is common in Christian designs and appears throughout history on many shields, coins, and cathedrals. It is sometimes hard to recognize, as it can be very stylized, often looking like a six-pointed asterisk with a small loop on upper right of the upright arm, or a highly ornamental X (see links below). The chi rho became popular in this context when Roman emperor Constantine I chose it as his military standard, which came to be known as the labarum. The labarum went on to be adapted by later Christian Roman emperors and Byzantine rulers.
However it is perhaps more useful as a bit of marginalia, used in the pre-Christian era as an abbreviation for chrēston, meaning 'good', used by scribes to mark any particularly interesting or important passage in a manuscript. It sufficiently well-known that some coins of Ptolemy III Euergetes were marked with a Chi Rho to serve as a control mark, akin to the modern mint mark.
In many cases, particularly in illuminated manuscripts, the chi rho can be so stylized that it is hard to recognize, in some cases looking more like a swirly X, and often separated back out into 'XP'.
Asterisk-form chi rho
Separated X and P.
Recently the chi rho has been used by radical anti-Islamic groups. This is nowhere near becoming a common usage of the symbol, but if you plan to make use of this symbol, you should be aware that some hate-based groups use it in their publications.