Skill
A packaged set of instructions an agent loads when it hits a certain kind of task, so it does not have to be told twice.
What it tells you about a developer
A developer who has written skills for their team has turned their own judgment into something every agent on the team reuses. That is a leverage move, and it shows up more often in senior engineers than in junior ones.
A skill is a written procedure stored alongside a project: how this team deploys, how it writes tests, what its review rules are. The agent loads the skill when the task calls for it and follows it, which keeps the instructions out of the context window the rest of the time. Skills are ordinary text files, so they get reviewed and versioned like code.