Have you ever noticed the hanging lamps in the sanctuary and chapel whose candles are always burning?
When you see this lamp for a small box hanging from a nearby wall. This box is called an Aumbry and it holds the reserved sacrament, wafers and wine left over from Eucharist, to be used when visiting the sick and home-bound.
The adjacent candle is left burning so that anyone entering the church can tell quickly that “Jesus is here”, in the form of the consecrated sacrament.
A worshiper might acknowledge this by genuflecting or bowing upon entrance, often also making the sign of the cross. Many people find the presence of the reserved sacrament to be helpful in their own private prayer.