As one of the most core aspects of running a business, revenue is a measure of the amount of money coming into your business. Specifically, revenue is the total income from all sales of goods or services. This is calculated by multiplying the total number of goods or services by the price at which they're sold.
You might see the terms “sales”, “income” or even "top-line" being used, these are equivalent to "revenue". Revenue is usually made up of the income from "core" goods or services as well as additional "other" revenue such as interest, fees or commission for example.