Beta measures systematic risk, i.e. how the stock moves with the market (i.e. how correlated it is to the market). The beta in the alphalerts stocks KPIs is for 3 years, using monthly prices for the regression.
For example, AAPL has a beta of 1.20 today, which means it is more volatile than the rest of the market and has returned 120% of what the rest of the market has. Conversely, MSFT has a beta of 0.78 today, meaning it is less volatile than the rest of the market and provides 78% what the market does on average.