Beta is a measure of the volatility of an asset relative to the market. It measures the relation of between the returns from an asset respect to the entiere market.
Beta is just the slope coefficient of the fitted linear regression of the returns of the asset respect to the market.
Therefore:
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\(\beta = 1\) same volatility as the market
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\(\beta > 1\) more volatile than the market
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\(0 < \beta < 1\) less volatile than the market
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\(\beta = 0\) uncorrelated to the market
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\(\beta < 0\) negatively correlated to the market