The Jesus Prayer is a traditional Eastern Christian prayer, used to fulfill St. Paul's exhortation to "pray constantly". It can be prayed over and over and over, often in tune with one's breathing. When possible it should be prayed aloud, or at least with the lips moving. (This helps bring the physical body in line with the spirit; also one's body can be praying the prayer while one's mind thinks on other things, thus providing a kind of "prayer cover" to be able to think more clearly and carefully.)
The traditional prayer goes as follows:
Lord Jesus Christ, Son of the living God, have mercy on me, a sinner.
It can also be abbreviated Lord, have mercy or even just Jesus, depending on your circumstances and how you use it.
Lord Jesus Christ acknowledges Jesus of Nazareth as Lord and Messiah (Christ). Son of the living God acknowledges him as the Second Person of the Trinity. Have mercy on me asks that he be forgiving, understanding, and forbearing towards our sins. It also asks that he be generous to us. A sinner acknowledges that he is God and we are not: that we are imperfect, in need of improvement, and that we act, from time to time, with bad intentions.
An expanded version adds:
Lord
Jesus Christ, Son of the living God, glory to you.
Lord Jesus Christ,
Son of the living God, I put my trust in you.
Glory to you glorifies God, not only for those things which we recognize as good and majestic, but also for those "bad" things that happen. We must glorify God for our sufferings. Paradoxically, suffering is how God perfects us and heals us. If we fight suffering instead of accepting it, we will only delay our healing. This petition, in effect, says "Be glorified, and I know you are glorified, in whatever happens, good or bad."
I put my trust in you means that we let go and let God, to use a popular saying — we trust God, despite our fears and despite what we perceive, and trust that the circumstances are working for our greater good. We also are trusting him to take care of us and totally surrendering ourselves to Him.