An API key is a string of characters used to authenticate to the Platform API. To use the Platform API, you will need an API key.
An API key refers specifically to your account, and performs a similar function to your username and password. However, API keys can only be used with the Platform API, not with the Platform or any bunker. Also, API keys can be time-limited, and can be revoked immediately if they are ever compromised. It's therefore far safer to configure an API key in an API client than it would be to reveal your username and password.
You can create API keys for yourself by logging on to the Platform, and selecting Account Settings. Each API key comes in two parts, the public and the private part; you will need to securely record both parts of your API key. If you lose the private part, there is no way to recover it, though you can create a new API key to replace it.