What does the average "30Mbps" mean in the specifications? Is that the max it can write to a connected drive? Read speed from a drive? Average transfer speed ov

What does the average "30Mbps" mean in the specifications? Is that the max it can write to a connected drive? Read speed from a drive? Average transfer speed over the network? If yes to any of the above, then isn't this quite slow for anything other than a few small pics and documents?