The term "debt" refers to one of the two major categories of financing a business, the other being equity. The key distinction between debt and equity is that debt is the result of the corporation borrowing money, whereas equity is generally the result of the corporation issuing capital stock to an investor. There is no bright-line distinction between debt and equity, but debt has a number of features that tend to differ from equity.
First, debt ranks above equity, meaning that debt claims must be paid before any equity claims are paid. Another way of saying this is that equity claims are subordinated to debt claims.
Second, debt typically offers a fixed return, such as 8% of the principal amount, rather than a variable return that depends on the company's performance. This fixed return is referred to as "interest," as opposed to the dividends that are paid on equity.
Third, debt tends to have a maturity date, at which the principal of the debt is required to be paid.
Fourth, debt must be "repaid" whereas equity generally is not required to be "repaid" (an exception is the case of stock subject to redemption). The fact that debt must be repaid means that the holder of debt (called a creditor) may have the ability to force the company into bankruptcy if the debt is not paid. In contrast, the failure to pay equity claims (even if they were fixed dividends on preferred stock) would not have the same result.