In the context of investing, beta is a measure of volatility of an investment relative to a benchmark.
Consider a portfolio of large-cap US stocks. If this portfolio has larger price swings (volatility) than the S&P500, it will have a beta above 1. High beta means you have to be more careful when you buy/sell as the price fluctuates more. A beta of 1 indicates the portfolio has the same amount of price fluctuation as the S&P 500. A small beta below 1 indicates more stable pricing.
Nerdwallet: What is a stock's beta?
Investopedia article on beta