A merchant account integration (or sales channel) refers to a connected POS/e-commerce service provider. 

For instance, if I have a Square account connected to Thrive, I have a Square "integration". Each Square merchant location would be a separate integration.

If I add a Shopify web store to that location, I now have two integrations at that same location.

On the technical side, an integration is specifically a Merchant ID assigned by your merchant account provider linked to a specific pool of inventory.

Like one cash register in a single store.

One business may have multiple integrations for the same account if they have more than one location. An integration is required for Thrive to be able to provide sales information.