The reason behind this premium stems from the risk-return tradeoff, in which a higher rate of return is required to entice investors to take on riskier investments. The risk-free rate in the market is often quoted as the rate on longer-term government bonds, which are considered risk free because of the low chance that the government will default on its loans. On the other hand, an investment in stocks is far less guaranteed, as companies regularly suffer downturns or go out of business.
If the return on a stock is 15% and the risk-free rate over the same period is 7%, the equity-risk premium would be 8% for this stock over that period of time.