A main water shutoff valve is the valve on the main supply line, usually where the line enters the house or beside the water meter, that stops water to the entire home. Most are quarter-turn ball valves or older multi-turn gate valves, and homes on a private well typically have one near the pressure tank.
Why it matters to you
When a pipe bursts or a water heater leaks, the minutes it takes to find and close this valve decide how much drywall and flooring get soaked.
Practical guidance
Find and label the main shutoff before you need it, and make sure everyone in the house knows where it is. Close and reopen an old gate valve gently once a year; if it is seized or leaks at the stem, have it replaced with a ball valve instead of forcing it.