The theory that stock price changes have the same distribution and are independent of each other, so the past movement or trend of a stock price or market cannot be used to predict its future movement.






Taobiz explains Random Walk Theory
In short, this is the idea that stocks take a random and unpredictable path. A follower of the random walk theory believes it's impossible to outperform the market without assuming additional risk. Critics of the theory, however, contend that stocks do maintain price trends over time - in other words, that it is possible to outperform the market by carefully selecting entry and exit points for equity investments.

This theory raised a lot of eyebrows in 1973 when author Burton Malkiel wrote "A Random Walk Down Wall Street", which remains on the top-seller list for finance books.