Mr. Market is the analogy given for the market. Imagine that you co-owned a company with Mr. Market, a man who, each day whether he is a good mood or in a bad mood, will buy or sell his shares in the company.
The analogy is to make the point that irrational investment is a reality. There is no scientific formula that each investor applies, and no singular rational way of thinking. Investors are people, they get emotional, they are at time too optimistic, and at other times, too pessimistic.
As an intelligent investor, you should do your homework. Your goal is not to beat the market, but to look to buy when Mr. Market is underestimating the value of his shares, and to sell when Mr. Market overestimates it. That is, never compare yourself to others, always focus on perfecting a sound strategy and having faith in it for the long run.