Ubi Primum is an Apostolic Constitution from Pope Leo XIII. Now, Apostolic Constitutions are the highest form of a Papal Decree, the Encyclical is a step down followed by Exhortations and Letters. We have looked at many an Encyclical here and a couple Constitutions from Vatican II. This one focuses on the Laws, Rights and privileges of the Confraternity of the Most Holy Rosary. Now this is some sixteen paragraphs long.
It begins with a reminder of what the Rosary is and noting that Popes have published many Encyclicals about the Rosary over the year including Diutirni Temporis, published roughly a month ago which said that this Apostolic Constitution would be coming out. This document continues with various rules and regulations of how Confraternities should be formed and what they should do. The basic rules are that members of a confraternity should have their names written in the register of the confraternity and, if possible, go to Confession and Communion and say the Rosary on the day they are enrolled; possess a blessed rosary; pray the Rosary every day or at least once a week; and whenever possible, go to Confession and Communion on the first Sunday of every month and take part in the Rosary processions. This also includes some talk about indulgences on Wikipedia they listed what they are, in which members are granted a plenary indulgence, on the day of enrollment and on various feast days. They also are granted a plenary indulgence when the Rosary is prayed in Church, or in a Public Oratory, in a family (family Rosary), Religious Community, or Pious Association. Otherwise a partial indulgence is granted. This is if you want to get into indulgences which still exist in the Church you just can’t buy them anymore.
If you want to dig deeper into the document why not take some time and read it. Pope John Paul II did add the Luminious mysteries and that is the only real thing that has changed since the constitution was written.