skills
imagine a chef with a binder of recipe cards. they don't memorize every recipe. when you order pasta, they grab the pasta card. when you order a cake, they grab the cake card. a skill is a card. claude only opens the ones it needs for your task.
a skill is a folder with a SKILL.md instruction file plus optional scripts and assets. claude loads it on-demand based on what you ask. you can write your own (brand voice, code style, document templates) and claude will apply them automatically when relevant.