The stopcock, llave de paso in Spanish, is the valve that turns the water or gas supply to a home, or to part of it, on or off. Although the Spanish name includes the word for key, it has nothing to do with a lock: it is a handle or a tap that you turn.
It is usually next to the meter, in the kitchen, in the bathroom or in a cupboard by the front door. Many homes have a main one and others for individual rooms.
In communities of owners you will also hear the term used informally for the shared key that opens the communal doors, such as the main entrance to the block or the way into the garages and storerooms.
A tip for coastal homes that stay shut up for months: before you leave, turn off the water at the stopcock. A leak in an empty property can go on for weeks without anyone noticing.