A rosary is an instrument to help us meditate on the life of Jesus.
Keep in mind that printed books are a pretty recent development in the 200 years of Christian history!
We are so accustomed to having a personal bible (even electronic copy now) that it’s hard for us to imagine what it was like to have to internalize the word of God by just hearing it proclaimed at Church. It would take a monk years to hand copy the scriptures (and literacy was far less common) so there had to be supplemental ways for people to memorize and contemplate the content of their faith that was more widely available to the rank and file believer. This is why artwork, statuary, stained glass, music, and devotional forms of prayer developed. The rosary is a devotional prayer, that when prayed properly, can unfold for a person virtually all the important events in the life of our Lord Jesus Christ. It is a summary of the Gospel, and therefore a way to get to know Jesus from a very privileged vantage point, through the eyes of His Mother, who was in some cases the only witness, or one of the only witnesses to these mysterious event. (For example the Annunciation by the Angel Gabriel, the Birth and death of Jesus.
PART I: THE SIGN OF THE CROSS
The prayer of the rosary customarily begins and ends with the cross of Christ. So taking the cross or crucifix in hand the sign of the cross is made from the forehead to the chest and from the left shoulder to the right. Meanwhile, the three Divine persons of the Holy Trinity are invoked by saying “In the name of the Father and of the Son and of the Holy Spirit, Amen.”
One of the reasons Christians made the sign of the cross in the early church was as a public witness. Imagine the holy martyrs who’s deaths were used as entertainment by the pagan Romans, When they couldn’t be heard, they could at least be seen by the crowds to be making the sign of Christ’s cross proclaiming their faith. It also recalls our Baptism into the Body of Christ in the name of the Trinity. It is a very precious ancient custom, and it recollects us well for prayer.