When there's a consensus, everyone agrees on something. If you're going to a movie with friends, you need to reach a consensus about which movie everyone wants to see.
Ever notice how people disagree about just about everything, from who's the best baseball player to how high taxes should be? Whenever there's disagreement, there's no consensus: consensus means everyone is on the same page. When you're talking about all the people in the world, it's hard to find a consensus on anything. There are just too many opinions. However, in a smaller group, reaching a consensus is possible.