An analogy I like to use is making tomato sauce using canned tomato paste. Let's say a given recipe calls for 50 ounces of tomato paste and they come in 10oz cans. A high efficiency would be like emptying five cans completely. A poor efficiency is like leaving a lot of paste sticking to the sides of the can to the point where you have to open a sixth can in order to get 50 ounces total. Basically, poor efficiency means you get a lower gravity for a given amount of grain used. Again, bang for your buck.