It's the description that gets inserted into the context, and then if that sounds useful, the agent can opt to use the skill. I believe (but I'm not sure) that the agent chooses what context to pass into the subagent, which gets that context along with the skill's context (the stuff in the Markdown file and the rest of the files in the FS).

This may all be very wrong, though, as it's mostly conjecture from the little I've worked with skills.