A Skill is a saved set of instructions for one repeatable job. Think of it as an onboarding note for a single task: do it like this, in this order, with these templates. Claude reaches for the matching Skill automatically when your request fits its description, so you just ask in plain words.
Ready-made document Skills (Word, Excel, PowerPoint, PDF) come built in; you can also make your own. Use one where a task has a right way you repeat; for a one-off, a plain prompt is simpler.