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.

Claude also has custom slash-commands, so you can force skill usage as you see fit.

This lets you trigger a skill with '/foo' in a way that resembles the way you'd use the command line.

Claude Code is very good at using well-defined skills without a command though, but in a scenario where this is some nuance between similar skills they are useful.