As St. Paul wrote in Philippians 2, "At the name of Jesus every knee should bow, of things in heaven, and things in earth, and things under the earth; and every tongue should confess that Jesus Christ is Lord." Very early on, Christians came to understand that the very name of Jesus had great power, and the recitation of His Name was itself a form of prayer.
This short prayer is a combination of that early Christian practice and the prayer offered by the publican in the parable of the pharisee and the publican (Luke 18:9-14). It is perhaps the most popular prayer among Eastern Christians, both Orthodox and Catholic, who recite it using prayer ropes that are similar to Western rosaries.
St. Paul urges us to "pray without ceasing," and this prayer is one of the best ways to start doing so. It takes only a few minutes to memorize, after which we can recite it whenever we remember to do so. If we fill the otherwise wasted moments of our day with the Holy Name of Jesus, we'll keep our thoughts focused on Him and grow in His grace.
The Jesus Prayer
O Lord Jesus Christ, Son of God, have mercy on me, a sinner.