Wright brothers: focus on wings or controls or engines! you only have room for one!

Apple: focus on hardware or software! Macs or iPods. You only have room for one.

Edison: focus on lights or electricity! You only have room for one.

Uber: focus on the driver network or the app! You only have room for one.

It's not a hard and fast rule. The point of focus is to get you to prioritize what important, but something you need to do multiple things.