The Chi Rho is a Christian symbol that looks like an X P. Pronounced Kie - Roe, it often appears to be a capital P overlaid on an x or t.
The Chi Rho is actually two Greek letters, the Chi, which corresponds roughly to the English letter C (or Ch) and the Rho, which corresponds roughly to the English R. These two letters are the first letters in the Greek word Christ. The Chi Rho is thus a monogram for Christ.
Some believe that the Chi Rho actually developed into the Celtic Cross, as seen in the image below (from John Allen's Early Christian Symbolism). For some history behind the Chi Rho, be sure to read about the Labarum.
Keep your eyes open! The Chi Rho appears in many places. Due to its small size, it is often incorporated in other images.
The Greek word "Christ" is the rough equivalent of the concept of Messiah or "Anointed one."
With this in mind, read Hannah's prayer in I Samuel 2.2-10. Then read Psalm 132.13-18. What do passages like this teach us about characteristics of the Lord's Anointed?
Next read Isaiah 61.1-4, and Luke 4.16-23. Who equates Jesus with the Messiah or Anointed one in this passage? What does it mean in this context?
Finally, read Mark 8.27-29. Who claims the title Messiah or Christ for Jesus in this passage?
One of the easiest ways to preach using the Chi Rho is to highlight the concept of "Messiah" or "anointed one." Pairing the passages from Isaiah 61 and Luke 4 make for a great connection to this symbol, and allow it to stand for themes around social justice. Paring it instead with the passage from Luke 9 makes the Chi Rho a powerful symbol of faith and evangelism.
The Chi Rho fits any time you want to talk about Jesus as Christ or Messiah. As proof of how versatile this image is, look at where the above passages appear in various lectionaries.
Isaiah 61 appears early in Advent.
Luke 4 appears during Epiphany.
Mark 8 appears in Lent.
The Chi Rho is an easy symbol to mention throughout the liturgical year.