Beta (or the 'beta coefficient') is a way to measure the relative riskiness of a share.
Beta (or the "beta coefficient") is a way to measure the relative risk (as measured by volatility) of a share or tracker fund. It does so by comparing the historic movement in the share price to that of the market as a whole.
So, for example, if a share typically moves up by 10% when the market has risen by 50%, beta is 10/50 = 0.2. Equally, a share that increased by 100% when the market rose 50% has a beta of 100/50 = 2. Therefore, a stock that swings more than the market over time has a beta above 1.0, while a stock that is less volatile than the market will have a beta below 1.0.
Shares with a high beta are generally more volatile and therefore seen as higher risk than those with a low beta such as utility companies.
See Tim Bennett's video tutorial: What is 'beta'?