Mala is a string of prayer beads used for reciting or chanting a mantra or other forms of spiritual exercise or sadhana, generally known as "japa/jaap". The mala beads are in sets of 27, 56 or 108 repetitions. A jaap/japa mala is very popular among Indians. A jaap mala is a very practical tool for meditation.
Rosary beads is used to keep the mind focussed on the meditation. It is a known fact that the mind tends to wander off all time, especially during the meditation practice. If your energy is low at the time of meditation, you can fall asleep. If the energy is too high, you can be distracted in other ways. During such times, the japa mala provides the much needed anchor. The prayer beads are moved in rhythm with the breath and the mantra, and hence both-sleep as well as excessive mental distraction are prevented by this action upon the beads.