Beta is a relative risk measurement, because it depicts a security's volatility against a benchmark. It's common that professionals calculate betas for stocks using the S&P 500 index as the benchmark. You should calculate betas using a security's best-fit index.
Beta is fairly easy to interpret. The higher a fund's beta, the more volatile it is relative to its benchmark.
The biggest drawback of beta is that it's really only useful when calculated against a relevant benchmark. If a security is being compared to an inappropriate benchmark, its beta is meaningless. Therefore when considering the beta of any security, you should examine another statistic: R-squared. The lower the R-squared, the less reliable beta is as a measure of the security's volatility. The closer to 100 the R-squared is, the more meaningful the beta is. Unless a security's R-squared against the index is 75 or higher, disregard the beta.