The prayer aid is usually equipped with 41 or 64 beads, but some with 150 or even 300. The number 41 is a representation of the suffering of Christ while 64 is the number of years that St Mary spent on this earth.
Not only is it common among grandparents, but this practice dates back to the 3rd century, to St Anthony, the father of monasticism.
The great saint tied a leather rope every time he started praying “Kyrie Elesion” meaning Lord have mercy, but Satan dismantled the knots. Then, St. Anthony devised a way to tie the knots into the sign of the cross so that the devil could not untie the rope as he was incapable of bearing the sign of the cross.
The practice of moving pebbles as each prayer was prayed was also a practice of the early fathers. So these two prominent early practices bring to us the prayer beads & ropes that we know today.
So, how do we use it?
Each bead is one prayer
The faithful carry on praying the Lord’s Prayer, the Salutation to St Mary, Kyrie Elesion, the Jesus Prayer, & Egzio Meharene Kristos, & asking for Mercy for the sake of the saints.
Each prayer is done rhythmically, as the breaths or heartbeat of humans.
We recall “..the tax collector.. would not even raise his eyes to heaven but beat his breast & prayed, 'O God, be merciful to me a sinner.” & The Blind Man “Jesus, Son of David, have mercy on me!” Lk 18:13,38
This method of prayer helps us to recite prayers with more concentration, and engage our sense of touch while praying
To count our prayers & keep our prayer rule (ie. recite the Lord’s prayer 12 times)
Aid to praying unceasingly (1 Thess 5:17)
So, is this repetition vain repetition?
The vanity of repetition comes only when the purpose is vain, and the name of our Lord is above all names, repeating it all day to avoid harmful speech or guard us is not a vain purpose
Vain reasons would include, self righteousness or to seem prayerful before others as hypocrites.
Through these holy practices wars were fought and won. So we take the Holy Traditions that were passed on to us in word & deed & imitate our saint fathers in simplicity & victory.