Investing your money is one of the oldest passive income strategies on the books. Investing in stocks (or funds of stocks) means that you are becoming a (very) partial owner in the company whose stock you own. When you purchase a stock, your expectation is that the value of the stock will either increase or decrease, and your goal is to buy low and sell high. (This is what everyone is shouting about on the floor of the NYSE in movies about the world of high finance).
In equity valuation, residual income represents an economic earnings stream and valuation method for estimating the intrinsic value of a company's common stock. The residual income valuation model values a company as the sum of book value and the present value of expected future residual income. Residual income attempts to measure economic profit, which is the profit remaining after the deduction of opportunity costs for all sources of capital.