John —

A tabernacle is a special vault used to hold the Eucharist, the bread that has been consecrated during the Mass or liturgy, and which we believe has been "transubstantiated" into the Body and Blood of Christ (such that, while the appearance and physical properties remain the same, its essence is no longer bread but the Body and Blood of Christ).

To prevent its desecration, when it is left over from a liturgy, it's kept in the locked tabernacle, but is available for people to come and worship since we believe it to be Christ Himself and consider it worthy of worship and adoration.