He's talking about counties, which are spread across a diverse geographic region. As per the image there it looks like the clear majority of California's counties leaned republican for governor. But the counties that leaned democrat are very densely populated and so end up choosing for the entire state.
On a national level this is the reason for the existence of both the electoral college and the senate. Imagine we did not have the college or senate and there was one very heavily populated state with a large group of mostly homogenous voters. Suddenly that state would be deciding every single action for the entire nation. Whether this is a good or bad thing probably depends on your views on what representation means, but it's safe to say that people in many states would feel they lacked such - which was the very reason that we rebelled against the British Empire.