A tap controls water flow by using a washer, cartridge, ceramic disc or similar valve that seals against the water supply when the handle is closed. If that seal wears out, becomes dirty or is damaged, water can pass through and drip from the spout, handle or base. Turn off the tap’s isolation valve or household water supply before checking it, and avoid overtightening the handle, as this can cause further damage. Replacing the worn part may fix a simple leak, but a licensed plumber can identify hidden problems or leaks inside the wall.