Beta is the regression coefficient of your returns against a benchmark — usually SPY for US equity traders. A beta of 1.0 means your portfolio moves dollar-for-dollar with the benchmark; a beta of 1.5 means it swings 1.5× as much in either direction.
Practical use: if you're running long-biased equity strategies, beta tells you how much of your P&L is just market exposure (which you could replicate by buying SPY) versus actual stock selection. A long-only fund with a beta of 1.2 in a bull market hasn't necessarily demonstrated skill — it's just been levered to the index.
Beta also flags hidden leverage. A portfolio of two-times-levered ETFs sized at 100% of account has an effective beta of 2.0 and the daily P&L swings to match. Most retail blowups happen not because of bad picks but because the implicit beta of the book was much higher than the trader realized.
Beta is sample-dependent and unstable. The same stock can have a beta of 0.8 over one year and 1.4 over the next — particularly true for cyclicals, regional banks, and small caps. Beta computed on a single quiet bull market is a lower bound on what you'll see in the next regime change.