The recommendation, as the name suggest, is the recommendation that you want to make to the user.
The recommendation is mainly the object for book keeping of the information around the recommendation. It contains the description that you want to show to the user, an image, labels for the buttons etc. It also has a lookup to a Flow. The flow is executed when the recommendation is accepted by the user. And in the flow you can then perform the steps necessary for that recommendation. For example, if you offer a product to the user then accepting it would mean that the Flow would create the order for the product, link it to the right user etc.