Hanuman Chalisa is a devotional song based on Lord Hanuman as the model devotee. It is a poem written by Tulsidas in the Awadhi language, and is his best known Hindu text apart from the Ramcharitmanas.The word "chālisā" is derived from "chālis" in Hindi, which means 40, as the Hanuman Chalisa has 40 verses.
Tulsi Das went to meet the then Emperor Akbar in Delhi after having had a darshan or vision of God Sri Rama in Gokul, the birth place of God Sri Krishna. The Emperor challenged Tulsi Das to show Sri Rama to him. When Tulsi Das replied that it was not possible without true devotion to Rama, he was imprisoned. In the prison, Tulsi Das wrote the beautiful verses of the Hanuman Chalisa on Sri Hanuman.
When he completed the Hanuman Chalisa in prison, an army of monkeys menaced the city of Delhi. The Emperor unsuccessfully tried to control the monkeys with his forces. Finally, the Emperor realised that the monkey menace was a manifestation of the wrath of Hanuman, the Monkey God. He released Tulsi Das and the monkeys stopped their mischiefs immediately after Tulsi's release.
Tulsi Das says in the Chalisa that whoever chants it with full devotion to Hanuman, will have Hanuman's grace. Amongst the Hindus of Northern India, it is a very popular belief that chanting the Hanuman Chalisa invokes Hanuman's divine intervention in grave problems, including those concerning evil spirits and this belief is based on the claim made in the Chalisa itself.