Beta is a measure of a security or portfolio's volatility relative to the volatility of the market as a whole. If a security or portfolio has a beta of 2.0 this would suggest that the security or portfolio is twice as volatile as the market. Thus, a market movement of 1% would suggest that the security or portfolio would move by 2%. A beta of less than one suggest that the security or portfolio is less volatile than the market.
Statistically, beta is defined as the slope of the linear regression of historical portfolio returns (dependent variable) in terms of historical market returns (independent variable).
The widespread acceptance of beta as a measure of systematic risk has been criticized by many including InnovationOwl.