An API Key is a way for a user or developer to grant access to the API for a specific program. Put simply, it's a kind of password you can use to allow certain programs to talk to other programs. Just like any password, you should be very careful to keep it secret. If you're not on the developer side of things, you probably won't have to interact with any of this, but just in case, this article should provide some clarification.
You can find the API Keys page under Settings in the admin panel. This section allows you to get API keys for using the Modern Retail API. https://api.modernretail.com/docs/
The first pair of keys in the list are the main two, and cannot be deleted or changed, because they're used for integration work by Modern Retail.
If you need additional API Keys for your agency, you can release additional keys from this page. You can also enable or disable keys at will from here.
If you have any questions about how the API Keys page works, don't hesitate to send a support request.