The hypothesis that all assets are priced correctly by the market that that no bargains exist. It is based upon the idea that the market has all the information relevant to price the asset properly.
Technology, especially faster communication, fed into the idea that market participants have all information available to them. Hence, there is no way to operate without the knowledge of everyone else in the market.
This also leads to the belief that nobody can "beat" the market long term. The fact that assets are traded at their fair market value, according to the theory, means the ability to outperform is not present.