Altar bell     The most stunning uses of church bells is their ringing at the elevation of the Host and the elevation of the Chalice in the Mass, an act that announces to the entire world that a   miracle is taking place  . Later this typically came, in most places, to be done only by a small handbell (the "Altar bell" or "Sanctus bell") inside the Church, but many places retain use of the large bells at this time. It's an exquisite moment (there are no words, really) -- one that would compel one to kneel if one weren't already kneeling. In modern times its use in the Mass is considered optional to the Consecration. Many parishes do not use them, but  they are recommended, and some do continue to use them. (It is at the discretion of the individual priest, unless directed otherwise by the hierarchy). In traditional Catholicism, according to the rubrics, the Altar Bell is rung at the Sanctus and at the elevation of both Species. The Chimes of the Trinity is the ringing of the bell three times to represent the Father,  Son, and Holy Spirit
The four bells represent  (Air, Water, Earth, Fire)