A logbook service is a scheduled service carried out to the manufacturer's specification, and it is the single biggest thing you can do to protect your car's value. Skipping it can void a new-car warranty and makes a used car much harder to sell later.
Beyond an oil and filter change, a logbook service checks fluids, brakes, belts, lights, suspension and the computer for faults. The exact list changes at each interval — some services are minor, others are major and cost considerably more. A workshop that stamps the logbook is confirming the work was done to spec.
You don't have to use a dealership to keep your warranty in most markets, as long as the servicing is done to the manufacturer's schedule with appropriate parts and the logbook is stamped. Independent workshops are often cheaper for the same job. Keep every invoice — a stamped book plus receipts is the strongest resale proof you can have.