A house worth a million with a nine hundred thousand mortgage is a very different deal from the same house with no mortgage at all. Companies work the same way. Debt-to-equity tells you how mortgaged the business is.
The math is simple. Take what the company has borrowed and divide by what the owners have put in. The plain question is this. For every dollar the owners have in the business, how many dollars do lenders have at risk?
Debt itself is not evil. A loan that builds a factory which earns more than the interest costs is good business. The danger is a bad year. Lenders must be paid even when customers stop coming. That is why cautious readers start asking hard questions when the ratio climbs well above 2 for ordinary companies. Heavy industries run higher by design, so always compare a company with its own kind.
Two companions complete the picture. Net debt is total debt minus ready cash. It respects the cash sitting in the safe. Interest coverage asks how many times the operating profit covers the interest bill. Above 5 is comfortable for most businesses. Below 1 means the business does not earn enough to pay its lenders.
Pocket rule: Before asking whether a company can grow, ask whether it can survive a bad year.