The beta of a share is a number describing the relation of its returns with that of the financial market as a whole.
This volatility measure gives you an idea how far the stock will fall if the market decreases and how high the stock will rise if the market increases.
A stock with a beta greater than 1 is considered more volatile than the market, less than 1 means less volatile. If the stock is perfectly correlated with the market, the beta equals 1.
If a stock gets a beta of 1.15, it has a history of fluctuating 15% more than the market. If the market goes up, the stock should outperform by 15%. If the market heads lower, the stock should fall by 15% more.