What Service history means for you
For a buyer, a service history shows the car has been looked after. Ask for the service book with garage stamps and any invoices, then check the dates and mileage against the MOT history. Gaps, missing stamps or mileage that doesn't add up are worth questioning, and a drop in mileage between MOTs is the classic sign of clocking.
For an owner, keep every invoice and stamp together. A free check on GetFreeCarCheck shows MOT history and mileage at each test, so you can see whether your records line up with the official ones.