In addition to Tantum’s excellent answer, I would like to add that the Rosary is a beautiful Christ centered set of prayers. In addition to the 15 traditional mysteries mentioned, Pope John Paul II added an additional five mysteries called they mysteries of Light or luminous mysteries-- Christ’s baptism, the wedding at Cana, the proclamation of the kingdom, the transfiguration, and the institution of the Eucharist (at the Last Supper). Of the twenty mysteries, 18 of them are about the Lord and 2 of them are what the Lord did for the Blessed Virgin Mary.
The Rosary uses the ancient practice of saying easy to remember prayers to draw you into deep contemplative prayer. It is absolutely not “babbling like pagans” as is sometimes claimed. There is a historical context to Jesus’ remark about us not babbling like pagans that few people seem to be aware of. Back in Jesus’ time, there were pagans who literally thought the more words they used in a prayer, the more likely it was that their god would hear their prayers. This is not what the Rosary is about. It is about being drawn into a deeper union with God by contemplating the 20 mysteries.