This is one of those things where once you get it you are going to be like "oh duh...". Ok so lets say you own a business, you are going to have a lot of money coming in from providing services/goods (revenue) as well as money going out paying workers, operating costs, and raw materials (expenses). So when you look at the cash flow to the business you are going to have a bunch of money coming in and money going out, but what is important is to know how much money you actually have. So when you "balance the books" you take all of the money coming in, you add it to what you already had, and then you subtract how much you have paid out. This gives you how much money you currently have.