Beta expresses the behavior of a security relative to its reference market. Statistically, beta is the slope of the line through a regression of data points representing the returns on securities relative to their market.
A beta equal to 1 means that the security is moving perfectly in-line with the market. A beta greater than 1 indicates a volatilile security with a lot of price fluctuations. A security with a beta less than 1 is more conservative and has fewer fluctuations than the overall market.