Something that's adjustable can be changed or altered so that it works or fits better. A car's adjustable seat can accommodate both very tall and very short drivers.
Most belts are adjustable — you can make them larger or smaller depending on the size of your waist. Seat belts and ball caps are also adjustable, so that they conform to different bodies and heads. When a loan or mortgage from a bank is described as adjustable, it means that the interest rate changes periodically. The verb adjust came first, from the Old French ajouter, "to join," based on the Late Latin adiuxtare, "to bring near."