Top round one of the major subprimals of the beef round primal cut.
Beef round is a large primal cut consisting of well-exercised muscles from the leg and rump of the animal. But the top round (also called inside round, because it comes from the inside of the leg), is actually a good cut of meat for roasting, as long as it's done slowly at a low temperature.
Cooking it this way ensures that it comes out medium rare, thus preserving what little tenderness the top round has to begin with.
It's also important to slice it thinly. Roasted top round is ideal for making roast beef for sandwiches.
Top round can also be fabricated into steaks, usually referred to as top round steaks. I'd be less inclined to cook a top round steak over the grill, simply because there are so many better options.
Having said that, if I were stuck on an island with nothing but a thick slab of top round and a grill, I'd marinate it, grill it quickly over high heat and then slice it thinly against the grain. This particular preparation happens to be known as London broil, and if I served it to the natives, I suspect they'd be friendly — provided I sliced it thinly enough. Otherwise, all that chewing would undoubtedly make them restless.