What it is trying to describe is the psychology around the current price of the stock.
In candlestick charts for example, if you get what is called a Bearish Engulfing Candle (where the open is higher than the previous day's close and the close is lower than the previous day's open) at the top of an uptrend, this could mean that the top may have been reached and the bears are taking over the bulls.
A Bearish Engulfing candle is seen as a bearish reversal pattern, as the bulls start the day by opening the stock at a higher price than yesterday's close, but by the end of the day the bears have taken over as the price drops below yesterday's open. This reversal pattern can be even more pronounced and effective if it coincides with other chart indicators, such as an overbought momentum indicator.
If you want to learn more look up about the Psychology of the market and Candlestick Charting.