Japa meditation is a very interesting meditation type as it involves the using of mantras as a means of relaxing the mind. Everybody has chanted one way or another without knowing it, and japa meditation is about chanting a particular word, a mantra, over and over again.
Widely used by Buddhists and Hindus, but also by Dharmic religions, Japa meditation uses words such as Om (spelled Aommmm) in a repeated pattern over and over again which produces a vibration through the head, stimulating the brain. The word Om is usually used to describe God and life creator, the energy it emanates. This is what is called a mantra.
Group Japa meditations are very popular and believed to be very powerful too. The mantra is either whispered or said out loudly by everyone in the group, and this is said to enhance and deepen relaxation as the group energy is much stronger than of a one person alone.
Everybody can use the Japa meditation, be it a beginner or already advanced meditator. It is noticed that beginners in meditation prefer to use the silent variant of the Japa meditation type, rather than saying it out loudly. It is ok, and there is no right or wrong on this. Also any other mantra, besides Om can be used, as long as it brings about a positive feeling, an uplifting of mind and it is one that you enjoy using.