Q. 1. What is the purpose of the "Altar bells?"

A. 1. In the Latin Rite of the Catholic Church, an altar or sanctus bell is typically a small hand-held bell or set of bells. The primary reason for the use of such bells is to create a joyful noise to the Lord as a way to give thanks for the miracle taking place atop the altar. An ancillary function of the bells is to focus the attention of those attending the Mass that a supernatural event is taking place on the altar. Such bells are also commonly referred to as the Mass bell, sacring bell, Sacryn bell, saints' bell, sance-bell, or sanctus bell (or "bells", when there are two or more). They are kept on the credence table or some other convenient location within the sanctuary.

Usage during today's Mass.

"A little before the Consecration, when appropriate, a server rings a bell as a signal to the faithful. According to local custom, the server also rings the bell as the priest shows the host and then the chalice." The usual moment chosen for giving the signal of the approach of the Consecration is when the priest stretches out his hands over the host and the chalice while reciting the epiclesis, but another moment traditionally used for this purpose is at the first words of the Sanctus. Pre-1970 editions of the Roman Missal, prescribed the choice of the Sanctus as the moment for the signal, but ringing the bell at both the Sanctus and the epiclesis was common practice in the first half of the twentieth century. They also prescribed a triple ringing of the bell at each showing of the consecrated elements.

[Source: https://en.wikipedia.org/wiki/Altar_bell]