It measures a fund's volatility compared to that of a benchmark. Beta < 1 indicates fund is less volatile than market. For eg- a beta of 1.5 would mean that for every 10% upside or downside, the fund's NAV would move 15% in the respective direction. Similarly, for beta of 0.8 every 10% upside or downside would indicate the fund's NAV to move 8% in the respective direction.