Beta indicates the proportion of the yield of a portfolio to the yield of the entire market (as indicated by some index). If there is an increase in the yield of the market, the yield of the individual portfolio may also go up. If the index goes up by 1.5% and the yield of your portfolio goes up by 0.9%, t he beta is 0.9/1.5 i.e 0.6. In other words, beta indicates that for every 1 % increase in the market yield, the yield of the portfolio goes up by 0.6%. High beta shares do move higher than the market when the market rises and the yield of the fund declines more than the yield of the market when the market falls.
A beta for a stock is derived from historical data. This means it has no predictive value for the future, but it does show that if the stock continues to have the same price patterns relative to the market in general as it has in the past, you've got a way of knowing how your portfolio will perform in relation to the market. And with a portfolio with an average beta of 1, you can create your own index fund since you'll move more or less in tandem with the market.