Beta is a crucial metric in finance used to assess an investment's risk and expected return relative to the overall market. It is calculated as the slope of the line in a scatterplot that compares the returns of an individual stock to the returns of the market benchmark, typically represented by a broad market index. Beta quantifies how much a stock's price moves in response to market movements, providing insight into its volatility and systematic risk.

A beta of one indicates that the stock's price tends to align with the market. When the market rises or falls, the stock with a beta of one is expected to exhibit similar percentage changes. A beta greater than one signifies that the stock is more volatile than the market, meaning its price movements are amplified compared to the overall market. For instance, if the market increases by a certain percentage, a stock with a beta greater than one would increase more significantly. Conversely, if the market decreases, the stock would also experience a more substantial decline.

On the other hand, a beta less than one indicates that the stock is less volatile than the market. Such stocks tend to experience smaller price fluctuations relative to the overall market movements, making them more appealing to risk-averse investors who prefer stability over high returns. These stocks are considered defensive investments often sought after during market uncertainty or economic downturns.

Beta is instrumental for investors in evaluating the risk a particular stock contributes to their portfolio. It helps understand how an individual stock will behave in different market conditions. Investors use beta to construct and balance their portfolios by combining stocks with different beta values to tailor the portfolio's risk level to their investment goals and risk tolerance. For instance, a balanced portfolio might include a mix of high-beta stocks for growth potential and low-beta stocks for stability.

Beta is based on historical data and assumes that past market behavior will continue, which may not always be the case. It also does not account for unsystematic risk—specific risks related to individual companies or industries. Therefore, while beta is a valuable tool for assessing systematic risk, it should be used alongside other risk assessment metrics and qualitative analyses.