This is used to describe the performance of the stock market. A 'bull market' is a different way to say that the stock market is 'performing very well'. When the stock market is a 'bull market', it means the overall average value of shares or stocks is increasing/growing quickly.
For example, 'we're in the middle of a bull market at the moment. the average value of shares has grown by 25% during this year'.
When the stock market is performing very badly (i.e. the average value/price of shares is falling/decreasing), the stock market is called a 'bear market'.
Between 2009 - 2014 the stock market has been a 'bull market' (sustained high growth in stock price values). During the stock market crisis of 2008, it wasn't. In this time it was a 'bear market' (falling stock market price values).
A place to buy bulls.
To learn more vocabulary connected to stocks and shares, you can do a free online exercise on stock market related vocabulary.