Debt-to-income ratio is the share of your gross monthly income consumed by required debt payments — loans, cards, financing.
Lenders use it as a capacity test: it answers whether you can absorb another obligation, independently of your credit history. It is also useful personally, because it puts a number on a feeling. A ratio creeping upward while income stays flat is an early warning that shows up long before a missed payment does, which makes it worth checking a couple of times a year rather than only when applying for credit.