The Brown Scapular is two small squares of brown wool connected by cords, that you wear so that one square sits over your chest, and the other between your shoulders - usually under your regular clothing.
It is a miniature version of the “Scapular” which is a part of the habit of the Carmelite Order. If you are "invested’ with it by being enrolled by a priest through a very brief ceremony, and you keep the promises you make during this ceremony (to pray the rosary every day and to honor Mary and keep chaste in life according to your state in life - whether single, married, or religious life), then you enjoy the special protection of the Blessed Mother, as well as enjoy the benefit of all the prayers and good works of the entire Carmelite Order, of which you become an honorary member, so to speak.
Millions of Catholics - like me - have done this, and it is highly recommended by the Church, including many popes.
Blessed John Paul II wore it all his life, and when he was shot in 1981, he asked the physicians specifically not to remove it from him, since there is a traditional story that the Blessed Mother told St. Simon Stock, to whom, according to tradition, the Scapular was revealed, that she would rescue from Purgatory anybody who died wearing it, having kept the promises.
I have benefited much from it, I can say.