Beta is a measure of a stock's volatility in relation to the market. By definition, the market has a beta of 1.0, and individual stocks are ranked according to how much they deviate from the market.
A stock that swings more than the market over time has a beta above 1.0.
If a stock moves less than the market, the stock's beta is less than 1.0.
High-beta stocks are supposed to be riskier but provide a potential for higher returns; low-beta stocks pose less risk but also lower returns. (Source: investopedia.com)