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It is an identifier so that an API you call knows who you are and can determine whether you have access to their service.

If you are writing a web app that needs to call an API someone else has written, they may have you request an API key from them. You'll then have the back-end of your app send that key to their service with each request. On their end, they'll match it up to your account, verify you have access to make the request, and determine what response to send back to you.

Its an password for your application to access it.

It’s a password like string generally for an API.

API Keys