Inside nearly every modern faucet is a cartridge or a washer-and-seat assembly that controls the flow of water, and that part wears down with years of use until it no longer seals fully in the off position.
A single steady drip is usually that worn part alone. A faucet that drips faster, or leaks from the base rather than the spout, can point to a failing O-ring instead, a different part with the same basic fix.
Mineral buildup, more common on a well-fed system, can also keep a valve from seating properly even when the part itself is not fully worn out, which is why the water source is worth mentioning when the problem is described.
The brand and model of the fixture matter too, since a cartridge from one manufacturer rarely fits another’s faucet body. Confirming that match ahead of the visit, where possible, means the right part is on hand instead of a second trip.