Is the method you use when you have some important to measure but you can't measure it directly e.g. Customer Satisfaction
So what you do? You look for things that are directly correlated to this thing and measure them. The more tightly they are connected to the original thing and the ore they are, the more accurate representation you will have
For example; you can measure the customers' satisfaction by measuring the number of returns/refunds, the number of complaint calls etc.
Know that this won't give you a 100% perfect measurement, but if you do it right it will give you pretty good representative numbers