Debt-to-Income (DTI) ratio measures how much of your gross monthly income is consumed by debt payments, including the proposed mortgage. Lenders look at both the front-end DTI (housing only) and the back-end DTI (housing plus all other debt).
Most conventional loans cap back-end DTI at 45%, FHA at 43% (up to 50%+ with compensating factors), and VA uses a residual income test rather than a hard DTI cap. DSCR and bank-statement programs don't use personal DTI at all.
If your DTI is too high, you have three levers: reduce monthly debt (pay off a car, credit card), increase income (second job, side business), or buy less house. Most experienced loan officers can help you optimize before formally applying.