Mapbox is a feature that allow you to add maps in a list (your MapBox) when clicking on one of buttons below visibles almost everywhere on 17b.
Mapbox:
Maps in this list can be downloaded in one click, but can't be shared. This list is private.
Favorites:
When you add a map in this list it increase map's note. This list can be shared.
User
List:
When you add a map in this list it doesn't increase map's note but this list is still sharable.
All those lists can be accessed by clicking on each link "MY SELECTIONS" in left menu