'Sanctuary' is often the name given to the part of the church around the altar. It is considered the holiest place in a church and may house the tabernacle; an ornamental receptacle for the Eucharist. The sanctuary may feature a lamp or candle, which burns constantly to symbolise the continual presence of Christ. There is also a cross, and although one cross has no more significance than any other, the Sanctuary Cross is usually given special reverence - simply because of its location.
In a more general sense, a sanctuary is the entire church building.
Until the Reformation, fugitives were supposedly entitled to immunity from arrest, as long as they stayed within the confines of the sanctuary. Even grasping the church door handle gave sufficient asylum. At one time, the sanctuary extended beyond the church building - a radius of a mile from the church - and the area was marked with crosses. Some of these have survived and are known as Sanctuary Crosses.
The law no longer recognises a consecrated area as a safe haven for criminals, although it still functions as such in some movies. Police and immigration officers might hesitate from bursting into a sanctuary because they want to avoid the spectacle of a church raid. But they are within their legal right to enter, detain, and also prosecute anyone who attempts to harbour the criminal.
Nevertheless, the law cannot prevent anyone, criminal or not, from seeking spiritual protection in Christ. This is the real meaning of sanctuary.