The short answer
A skill is procedural knowledge stored as text. It contains a description that decides when the skill is loaded, and a body that tells the agent how to do a particular job. When your request matches the description, the body enters the agent's context and changes how it works. When it does not match, nothing happens, which is why the description matters more than the body.
Skill, MCP server or integration
A skill gives an agent a procedure. An MCP server gives it a capability, meaning access to a system it otherwise cannot reach, such as your database or an internal API. An integration connects two products. If you need the agent to know how you do something, write a skill. If you need it to be able to do something new, build a server.
Why some skills never trigger
The most common failure is a description written in the author's vocabulary. If the skill is described as "deck production workflow" and people ask for "a presentation", it will not load. Descriptions should be written the way the person asking would phrase the task, including the phrasings you did not think of first.
What belongs in a skill and what does not
Encode procedure, conventions, constraints and house specifics. Do not encode general knowledge the model already has, because that is padding that competes for context. The best skills usually capture the boring constraints somebody keeps rediscovering, such as an export format's real limits.
How many is too many
Skills stop being predictable when their descriptions start to overlap, because the agent then has to guess which one applies. The fix is deletion and consolidation, not another skill. A small set that always triggers correctly beats a large library nobody can predict.
The security part nobody mentions
Installing a skill means handing written instructions to an agent that can run commands on your machine. Directory listings are not reviews. Read the source of anything you install, and treat a skill from an unknown author exactly as you would treat a script from an unknown author.