Hi,
That is known as the sanctuary lamp and it doesn’t have to be red, although most are.
It is there to confirm that the Blessed Sacrament is present. So it is a very important light.
It’s actually a hold-over from Judaism. In orthodox synagogues, there is always a red lamp burning near the Torah, the book of the Old Law. Our sanctuary lamp signifies the fulfillment of the Torah, and His Presence in the sanctuary.
Fr. Vincent Serpa, O.P.
