The sanctuary lamp gives evidence that the Blessed Sacrament is reserved nearby. It signals the presence of the Lord Jesus in the Tabernacle and burns for seven days.
John 8:12
New Revised Standard Version (NRSV)
Jesus the Light of the World
12 Again Jesus spoke to them, saying, “I am the light of the world. Whoever follows me will never walk in darkness but will have the light of life.”