Debt-to-equity (D/E) tells you how much a business leans on borrowed money. A ratio of 0 means no debt. A ratio of 1 means it has as much debt as equity. Above 2 means it carries a lot of debt.
Lower is usually safer. Buffett-style quality businesses often run below 0.5. Their cash flows are strong enough that they do not need to borrow to keep running.
The industry matters a lot. Banks and utilities carry heavy debt by their very nature, with a D/E often between 5 and 10, while software companies should sit near zero. Always compare a company to others in its own industry, not across different ones.