A sanctuary lamp in a Catholic church signals the presence of Jesus in the Holy Eucharist in the tabernacle.
When entering a Catholic church, one of the most distinctive elements you will see is a sanctuary lamp.
The word “sanctuary” is generally defined as a location that is “holy,” “sacred,” or “set apart.”
In a Catholic church, this is the place where Mass is celebrated and where most of the time consecrated hosts are reserved in a tabernacle (though tabernacles are sometimes found in their own separate chapel, especially in large cathedral churches).
In order to alert the faithful that consecrated hosts are present in the tabernacle, sanctuary lamps were placed near them and remain perpetually lit.
Normally these lamps are enclosed in a red glass, but local traditions differ throughout the world.
The tradition of having a lit lamp reminding people of the presence of God can even be found in the Old Testament, as the Catholic Encyclopedia explains:
In the Old Testament God commanded that a lamp filled with the purest oil of olives should always burn in the Tabernacle of the Testimony without the veil (Exodus 27:20, 21).
Besides alerting the faithful of the presence of God in the tabernacle, the lamp also symbolically reminds the faithful of Jesus Christ as the “light of the world.”