Mary, the Mother of God, the Theotokos. Throughout Christian history Mary has held a special place in Catholic devotion. It isn't surprising that one of the first pieces of Catholic art was an icon of Mary painted by St. Luke. The Miraculous medal is the most popular Marian medal but there are all kinds of medals for other devotions including Our Lady of Guadalupe and La Madonna del Ghisallo, the Patroness of Cyclists.
Wearing a Marian or other patron saint medal is a sign that you have a special love for the saint and want to be reminded of their holiness and also to ask them for assistance as you would any other member of your family.