An API key is a secret code that helps an app identify itself when connecting to an online service. The service can use it to decide whether the app is allowed to make a request.
It is like a key to a locked room: anyone who gets a copy may be able to enter. Do not post it in a public place, include it in a public code project, or paste it into an untrusted chat.
Store API keys using the service's recommended secret storage, and replace one if you think someone else may have seen it.