The consumer key can be thought of as a username used to authenticate access to the tool.
It can be used by the tool provider to uniquely identify the Moodle site from which users launch into the tool.

The consumer key must be provided by the tool provider. The method of obtaining a consumer key varies between
tool providers. It may be an automated process, or it may require a dialogue with the tool provider.

Tools which do not require secure communication from Moodle and do not provide additional services (such as grade reporting)
may not require a resource key.