Market efficiency is a hot topic in finance, with experts trying to figure out if it's possible to make extra money by trading stocks using certain information. If markets are efficient, prices of stocks and other financial stuff are fair and accurate, making it hard to make extra cash. This idea is linked to the efficient market hypothesis, which comes in three forms based on the info available at a given time. On the flip side, some believe that prices of financial stuff have mistakes that can be found, letting smart investors make extra profits.