Mr. Market is an allegory created by investor Benjamin Graham. Since its introduction in Graham's 1949 book The Intelligent Investor, it has been cited many times to explain that the stock market tends to fluctuate, and that it is usually best to ignore these fluctuations when determining whether to buy or sell stocks. Graham instead believes that it is important to focus on whether the stock valuation of a company is reasonable after calculating its value through fundamental analysis.
Graham's favorite allegory is that of Mr. Market, an obliging fellow who turns up every day at the share holder's door offering to buy or sell his shares at a different price. Often, the price quoted by Mr. Market seems plausible, but sometimes it is ridiculous. The investor is free to either agree with his quoted price and trade with him, or ignore him completely. Mr. Market doesn't mind this, and will be back the following day to quote another price.
The point of this anecdote is that the investor should not regard the whims of Mr. Market as a determining factor in the value of the shares the investor owns. He should profit from market folly rather than participate in it. The investor is advised to concentrate on the real life performance of his companies and receiving dividends, rather than be too concerned with Mr. Market's often irrational behavior.
The Intelligent Investor by Benjamin Graham, first published in 1949, is a widely acclaimed book on value investing, an investment approach Graham began teaching at Columbia Business School in 1928 and subsequently refined with David Dodd. Famous investor Warren Buffett described it as "by far the best book on investing ever written", a sentiment echoed by other Graham disciples such as Irving Kahn and Walter Schloss.