In a Catholic Church, the sanctuary is the part up at the front. It is the part where the altar is and where the priest and the other people who are directly involved in celebrating Mass are.
Technically speaking, the main part of the typical church is divided into two parts. One part is the part where the congregation sits during Mass. This part is technically called the “nave” of the church. The other main part is the sanctuary.
In medieval times, sanctuaries were very clearly separated from the nave. There was something called a “rood screen” that physically separated the two. This was meant to emphasize that the sanctuary was even holier than the rest of the church. After rood screens went out of fashion, there was typically a low “altar rail” that separated the sanctuary and the nave. Communicants would kneel there to receive the host. Today, most churches do not have anything actually separating the nave and the sanctuary. However, the sanctuary is typically slightly raised as a way of designating that it is a separate part of the church.