A skill is a small package for one job. It holds the steps, plus anything the job needs: an example of a good result, a template, a script. Teach the job once and ChatGPT does it your way from then on.
Once installed, ChatGPT can pick a skill on its own when your request matches the skill's description, or you call it with an @mention. You can create one by chat (with the built-in skill-creator), in the editor, or by uploading a file, and share it with colleagues. Skills are on Business, Enterprise, Edu, and Healthcare, and often ship inside a plugin.