The sanskrit word "kirtana" means "to repeat". The practice of Kirtan is about repeating the name of the Divine with a feeling of love and devotion.
Devotional chanting is a central practice in Bhakti Yoga, one of the four classical paths of Yoga.
While chanting, the practitioner can transform his emotions in devotion, and put aside the mind to open the heart and gain access to a profound and yet very natural state of inner bliss.
Kirtan constitutes an excellent mean to sooth the nerves and to calm down the mind, which will then be naturally inclined to meditate.
Kirtan is originally practiced with Sanskrit mantras but it can really be done in any languages, as long as the feeling and mental attitude (Bhava) is there.
For westerners who practice devotional chanting for the first time, it can sometimes feel a little awkward in the beginning. But as soon as this phase is passed, the first effects can be felt quite quickly and the practitioner can really feel a deep inner joy radiating from the heart center, a kind of feeling the person probably had never experienced before.
Kirtan according to Swami Sivananda
"Kirtan is the easiest and most direct path to God. Kirtan opens the heart, fills the mind with purity and harmony, and creates a longing for the Divine. He who sings the name of God forgets about his body and the world. One can experience a state of supra-consciousness when singing kirtans with devotion. Singing kirtans in sanskrit, even mecanically, produces profound effects. When one adds devotion and understanding, benefits are immeasurable."
Extract of "Bhakti yoga" by Swami Sivananda