Kirtan is Sanskrit for "chant" or "glorify" — to chant the names of the Divine, or God. God has many names in many languages. It is through hearing and chanting that we connect bodymind with heart, and commune with the Divine. This is a deeply purifying practice of Bhakti Yoga, the yoga of devotion. When we engage ourselves, absorb ourselves in these transcendental names, we rise above the bondage of material nature.
One of my teachers says: 1. Always listen intensely, 2. Always chant back ("sing louder w/ your lousy voice"), 3. Always do it with a longing in your heart.
(Paramananda's instructions on Kirtan from the sadhu)