PITI stands for Principal, Interest, Taxes, and Insurance — the four components of a typical monthly mortgage payment. Some lenders also include HOA dues and PMI/MIP in the PITI calculation, in which case you'll see PITIA.
When a lender or calculator quotes a monthly payment, ask whether the number is just P&I or full PITI. The difference can be hundreds of dollars per month, especially in high-tax states.
DTI calculations use full PITI (plus HOA, plus PMI/MIP if applicable). When you're estimating affordability, always work with the full PITI number — not just the principal and interest.