1. Is it in the Bible?
Yes. (there are 171 references to "sanctuary" in the KJV Bible--ALL of them--even the 11 which are in the new testament--all refer in context to the old covenant)
2. What do we believe it means?
The sanctuary is the church building. It is the dwelling place of God's presence.
3. What does it really mean?
this idea, like the "House of God buzzword is an indication of how the church operates by old covenant concepts, which the new covenant clearly changes. the old covenant sanctuary was behind the veil of the temple in Jerusalem. god's presence in the earth was behind this veil and the offerings were made in this sanctuary. Because of these offerings, God cleansed and forgave Israel's sin. God accepted the payment of the blood and bodies of animals and the offering of grain (made into bread) in this sanctuary. Christians ought to know that Jesus is the offering for sin in the new covenant. he makes His perfect offering (His body--blood--bread) in the sanctuary of God in Heaven.
The new covenant makes it clear that Christians do not need to build a sanctuary for god. His dwelling place in the earth is in the heart of believers everywhere.
One more point to consider:
The tradition of a Christian "sanctuary" originates
in the dark ages catholic church. Catholics believe that Jesus
is literally present in the eucharist. The building of ornate
structures to house this "eucharistic presence" comes
from this belief. It is interesting to note how the protestant
church does not believe in the eucharistic presence, yet often
calls it's meeting structure a "sanctuary" and spends
every bit as much time, effort and money to construct this
"sanctuary building" as the catholics!