What is Beta?
Beta measures how much an asset's returns have tended to move with a market index. It is the slope of a regression of the asset's returns on the index's returns: a beta of 1.2 means the asset moved about 1.2% on average for each 1% move in the index. It describes the past relationship, which can change.
Why Beta matters
It separates the part of a holding's risk that comes from the market from the part that is its own.