Beta is the risk, relative to the stock market as a whole, an investor assumes by owning a particular stock. That's why you need to compare the returns of a single stock against the returns of an index. The index is the benchmark against which the stock is judged. The risk of an index is fixed at 1. A beta of lower than 1 means that the stock is less risky than the index to which it's being compared. A beta of higher than 1 means the stock is more risky than the index to which it's being compared.  Take this example: Say that the beta of Gino's Germ Exterminator is calculated at .5. Compared to the S&P 500, the benchmark to which Gino's is being compared, it's half as risky. If the S&P moves down 10%, Gino's stock price will tend to fall only 5%. As another example, imagine that Frank's Funeral Service has a beta of 1.5 when compared to the S&P. If the S&P falls 10%, expect Frank's stock price to fall more than the S&P, or about 15%. High risk, high reward; low risk, low reward. A stock with a low beta won't lose as much as the S&P when it falls, but it won't gain as much as the S&P when it posts gains. On the other hand, a stock with a beta over 1 will lose more than the S&P when it falls but will also gain more than the S&P when it posts a gain.  For example, pretend Vermeer's Venom Extraction has a beta of .5. When the stock market jumps 30%, Vermeer's only gains 15%. But when the stock market sheds 30%, Vermeer's drops only 15%. If you make your beta calculations and find out the stock you're analyzing has a beta of 1, it won't be any more or less risky than the index you used as a benchmark. The market goes up 2%, your stock goes up 2%; the market goes down 8%, your stock goes down 8%. A good mix of high- and low-beta stocks will help you weather any dramatic downturns that the market happens to take. Of course, because low-beta stocks generally underperform the stock market as a whole during a bull market, a good mix of betas will also mean you won't experience the highest of the highs when times are...

TL;DR in Vietnamese:
Tìm hiểu cách diễn giải hệ số beta. Nhận thức rằng rủi ro thường liên quan đến tỷ suất sinh lời. Hiểu rằng cổ phiếu có giá trị beta bằng 1 sẽ chuyển động theo sát với thị trường. Kết hợp cả cổ phiếu có hệ số beta cao và hệ số beta thấp vào danh mục đầu tư của bạn để đa dạng hóa hợp lý. Nhận thức rằng giống như hầu hết các công cụ dự báo tài chính, hệ số beta không thể dự đoán tương lai một cách đáng tin cậy.