Each development project require the developer working on it to get a unique set of API keys (client ID and a client secret) to get authorised correctly.
In the developer site you create an app for this. The two most common type of apps are:
Showing product information on your website
You can let users read all the information about your products, see what files and file formats are available to download. This is the equivalent to browsing for objects on bimobject.com without being logged in. (Client Credential is needed)
Allowing users to download file content
To download any product files or content from bimobject.com a user needs to be logged in. So to set this up on your website you need to require users to have a BIMobject account and to log in. (Authorization Code with Proof Key is needed)
Depending on what you are developing it is important that you pick the right type of authentication - please read more in the API documentation.
After filling out the needed information you click Create to get you unique Client ID and Client Secret.