From Traderpedia

Definition: An investor or trader who does the opposite of the majority at any particular time.

In other words, if most investors are buying, the contrarian will sell and vice versa. A contrarian view is that when the market is going up, most people are already or nearly fully invested and have no additional funds to make more purchases. Thus, if the market has not already reached its peak, it will do so in the near future. Conversely, when other investors believe the market will or is declining, they have already sold their investments. Thus, there are little or no sellers left and the market.

Contrarian views can also be taken with snake oil trade signal services - some of these services provide market calls so bad that doing the exact opposite of them can sometimes prove profitable.