The repeated-instruction problem
If you've ever explained the same procedure to an agent in more than one session, you've felt the problem a Skill is designed to solve. Maybe it's how your team wants commit messages formatted, how to fill out a specific internal form, or the exact sequence of steps to validate a data export before it ships. Each time a fresh session starts, that knowledge doesn't carry over, so you either re-type it, or the agent guesses and gets some detail wrong. Neither is sustainable once the number of such procedures grows past a handful.
A Skill is the fix: a reusable package, typically a folder containing a SKILL.md file, that captures a procedure once and makes it discoverable by an agent across sessions. Instead of the instructions living in your head or in a chat you have to re-paste, they live as a standing resource the agent can find and load when it's actually relevant. Critically, they aren't loaded all the time, only when the current task matches what the Skill is for, which is exactly what keeps this from becoming another form of instruction overload.