Efficient Market Hypothesis

After reading lots of financial models on how to valuate stocks, I read this theory about efficient markets which says that one cannot make excess gains in the stock market. Which actually means that for all the financial modelling one does to forecast the prices of stocks so as to buy them low and sell them high, the stocks may actually not behave as the models suggest. The theory uses the word "random" for stock movements. Yes, the theory says that as soon as or before a rational man realises that stock follows a particular trend and tries to make excess profit out it, there would be other rational men (or women) in the market who would have identified this trend already or along with him. Hence by the time you actually decide to buy or sell a stock to make an excess, again, the stock price would have already adjusted itself.