What an Agent Skill is
A folder of instructions your agent loads only when the task calls for it.
The problem
You explain the same standards to your agent again and again, and the results still vary. A prompt disappears when the session ends. A skill stays on disk and is ready the next time.
What it is
A skill is a folder with a SKILL.md file. The top of the file holds a name and a description. The agent reads the description to decide when the skill applies, and loads the full instructions only then.
Why it beats a saved prompt
A skill can carry steps, templates, scripts and checks together. It is also triggered by what you ask for, so you do not have to remember it exists.
Summary
Use a prompt for one-off work. Write or install a skill for anything you want done the same way twice.