In financial markets, speculators bet that the current price of a security is above or below its future value, and buy or sell according to the current price.
In an efficient market, prices on traded assets, e.g., stocks, bonds, or property, already reflect all known information. No investor can have an advantage in predicting a return on a stock price - since no one has access to information not already available to everyone.
Empirical market analyses have consistently found problems with the efficient market model, the most consistent being that stocks with low price-to-earnings outperform other stocks.