Rejoice always, pray without ceasing, in everything give thanks; for this is the will of God in Christ Jesus for you (1 Thessalonians 5:16-18).
Saint Paul, in writing to the Christian community in Thessaloniki (Thessalonica), directs them to “pray without ceasing.” The tradition of unceasing prayer is therefore of obvious Apostolic origin, and has been consistently followed by faithful Christians throughout the history of the Church.
Orthodox Christians practice unceasing prayer by reciting the “Jesus Prayer,” a form of invocation used by those practicing mental prayer, also known as “prayer of the heart.”
The words of the Jesus Prayer most usually said are “Κύριε Ἰησοῦ Χριστέ ἐλέησόν με - Lord Jesus Christ, have mercy on me.” There are slight variations in the form of the Jesus Prayer, the most common being “Lord Jesus Christ, Son of God, have mercy on me the sinner.”
To practice unceasing mental prayer as indicated in Holy Scripture, Orthodox Christians repeat the Jesus Prayer constantly and continually, regardless of whatever else they are doing. The purpose of the Jesus Prayer is to be continually united with God trough the unceasing remembrance of His presence and the repeated invocation of His name. By doing so, we will always serve Him and others with the virtues of Christ and the fruits of the Holy Spirit.
A prayer rope is commonly used when saying the Jesus Prayer, either to count the recitation of the Jesus Prayer a fixed number of times or simply as a reminder to repeat it continuously.
Click on the following links to read more about the Jesus Prayer and the Prayer Rope:
The Jesus Prayer (Greek Orthodox Archdiocese web page)
Using the Jesus Prayer (Antiochian Orthodox Archdiocese web page)
The Jesus Prayer (Orthodox Church in America web page)
Ten Steps to a Better Prayer Life (Antiochian Orthodox Archdiocese web page)
On prayer as the source from which all good comes (www.monachos.net web page)
Spiritual Nourishment For The Soul (By the V.Rev. Nektarios Serfes)