If something is adjustable, you can change it a little — move it, lengthen it, tighten it — so that it fits a different person or a different job. An adjustable chair goes up and down; an adjustable strap gets longer or shorter; an adjustable lamp turns where you need the light. The word is ordinary and practical, used on product labels and in shops. It does not mean "we already changed it"; it means "it is built so that you can change it". Though actually, it usually means small, useful changes, not making a new thing from the old one. A belt can be adjustable; a law is not, in everyday talk.