A prayer is something you say to a god or deity. Your prayer can be silent words of thanks or a plea for help that's spoken out loud.
Most religious believers say a prayer from time to time. While some people might use prayer as part of their plan for recovering from an illness, others bend their heads in prayer whenever they attend church or temple services. You can also say a non-religious prayer, just an entreaty or request for help. Prayer, from the Old French prier, has a Latin root: precaria, "obtained by prayer or given as a favor."