Mantra meditation is a spiritual and religious practice that has a place in practically all religious traditions, although the method of practice may differ. In the Vaishnava tradition (which includes the devotees of Krishna) this type of meditation has two basic forms. The first is individual practice and the second is congregational.
Individually devotees of Krishna perform a daily schedule of personal prayer and meditation. This is centred around the recitation of the names of Krishna (God) using prayer beads to count out the number of names chanted. The prayer, or "mantra", that they repeat is called the Hare Krishna Maha Mantra:
Hare Krishna Hare Krishna
Krishna Krishna Hare Hare
Hare Rama Hare Rama
Rama Rama Hare Hare
The second form of this process, which is congregational, includes the use of musical instruments. Instead of chanting on beads the mantra is sung. One person leads the singing while others in the group respond. Depending on the circumstances this religious performance is done either within temples of Krishna or in public. In India such public performances are very common and have been for many centuries.
"Hare", "Krishna" and "Rama" are all names used in the Vaishnava tradition that refer to God and His energies. Because God is spiritual and all-powerful if someone chants His name then they will become purified - materially and spiritually. This chanting is also considered a form of prayer whereby the devotee is appealing to the Lord to please engage the devotee in the Lord's service.