The main household shut-off valve is located on the main water line coming into the house. Its job is to either allow or stop water travelling through the supply pipes to all water fixtures. These shut-off valves can leak if the packing nut (under the valve handle) is loose, or if the packing washer (under the packing nut) is old and needs replacing. Carrying out these jobs takes very little time.
Turn the valve handle/lever counter-clockwise, so it is in the "open" position.
Place an adjustable wrench around the packing nut. Turn it clockwise in 1/4-turn increments, until the valve stops leaking. If the nut is fully tight and the valve still leaks, the packing washer will need to be replaced.
Turn the valve handle/lever clockwise, to the "closed" position.
Remove the valve handle/lever, by taking out its screw. Place the wrench around the packing nut and turn it counter-clockwise. Pull the packing washer off the valve stem, underneath the packing nut.
Place a new packing washer on the valve stem and thread the packing nut back on. Tighten it with the wrench. Reattach the valve handle/lever.
Turn the valve handle/lever to the "open" position. If the valve leaks, place the wrench around the packing nut and turn it clockwise in 1/4-turn increments, until the valve stops leaking.
Shut-off valves have either circular handles or small levers to turn them to "open" or "closed" positions. The handle/lever is attached to the valve with a screw.