The tabernacle is the chamber where leftover Eucharist is reserved for the sick or dying or to be used at a later mass.
If the consecrated body and/or blood of Christ is present inside there is a lit candle above or beside it. Most Catholic churches and some Episcopalian/Anglican churches observe this. Catholics genuflect before the tabernacle as a sign of reverence. Usually tabernacles are focused in a central location behind the altar although some have them off to the side on a side altar.