I'm currently learning for the 70-488 exam, and ran into this as one of the learning items: "Subscribe to published content types".
My interpretation of this is "How do you subscribe a site collection to the content type hub?"
If my interpretation is correct, I assume that a site collection becomes "subscribed" once it is associated to the managed metadata service that consumes content types from a content type hub.
Please correct me if I'm wrong, I'm a bit confused by this.