Alpha is a measure of risk-adjusted performance. An alpha is usually generated by regressing the security or mutual fund's excess return on the S&P 500 excess return. The beta adjusts for the risk (the slope coefficient). The alpha is the intercept.
Example: Suppose the mutual fund has a return of 25%, and the short-term interest rate is 5% (excess return is 20%). During the same time the market excess return is 9%. Suppose the beta of the mutual fund is 2.0 (twice as risky as the S&P 500). The expected excess return given the risk is 2 x 9%=18%. The actual excess return is 20%. Hence, the alpha is 2% or 200 basis points. Alpha is also known as the Jensen Index.