The Media Plan Recommender is part of the capabilities within the Research tab. It uses artificial intelligence to help you formulate better media plans by uncovering media programs you may not have otherwise considered. The Recommender also improves your productivity by automating the process of adding habitual placements to your media plan.
You can "train" the Recommender by building RFPs and media plans for each Advertiser, and rejecting media programs that aren't applicable.
A detailed explanation for how it works can be found here.
To learn how to use it, check out this page.