This is only relevant for individuals who want to create a connector.
This refers to how you define the data that you can retrieve from a Source. For example, if you want to retrieve information from an API, the data that you can receive needs to be defined clearly so that Airbyte can have a clear expectation of what endpoints are supported and what the objects that the streams return look like. This is represented as a sort of schema that Airbyte can interpret.
Learn more on Beginners Guide to Catalog.