The efficient market hypothesis argues that the price of widely-traded securities, like stocks, reflects an aggregation of all available information about them. This means investors can’t spot “deals” or “overpriced” assets and use that knowledge to outperform the average market return (without taking on more risk).
two economists who are walking along when one of them spots a $20 bill. They say: “Look! There's $20 on the ground!” The other replies: “Impossible! If it were really there, someone would have picked it up already.”