Back in the day, monks read all 150 Psalms each and every day and that practice began to extend to the general public. As most people couldn't read, they substituted the 150 psalms with 150 recitations of the Lord's Prayer, otherwise known as the Our Father (Pater Noster is "Our Father" in Latin). The string of beads, named after the prayer which is recited on it, helped the person keep track of how many they had prayed.