Typically, it is a repetitive meditative prayer. Simple forms of the prayer can be as short as “Lord Jesus Christ have mercy upon me a sinner.” Longer versions are common: “O Lord Jesus Christ, Son of God, have mercy on me a sinner!” The shortest is simply “Lord have Mercy!”
St. Luke’s Serbian Orthodox has a nice page on prayer ropes with several other prayers used with it:
Beside the Jesus prayer, we can recite a prayer to the Most Holy Mother of God, or to the saints: “O Most Holy Mother of God, save me - a sinner!”
“O Holy (name of the saint) pray to God for me - a sinner!”
With prayer ropes we can pray for our relatives, neighbors, or our people, by holding the first knot of our prayer rope and saying:
“O Lord Jesus Christ, have mercy on Thy servants (name, or a list of names), and with the rest of the knots we do not repeat again the names of those that we prayed for, but we say: " O Lord Jesus Christ, have mercy on Thy mentioned servants…!”
This way we can also pray for the departed relatives: " O Lord Jesus Christ, give rest to Thy servants (names), and again with the rest of the knots: " O Lord Jesus Christ, give rest to Thy mentioned servants!"
Russian Monastics use “Lord Jesus Christ, Son and Word of the living God, through the intercessions of thine all-pure Mother and of all thy Saints, have mercy and deliver us.” for each knot.
It serves the same purpose as a Rosary: to count the prayers while praying so one can meditate while praying and not have to count.
Next time you say the rosary, think not so much about the prayers, but the mysteries. Delay the prayers if you need to, but dwell deeply upon the mystery for that decade. Then pray the prayers of the decade.