PITI is shorthand for the four parts of a typical monthly mortgage payment: principal, interest, taxes, and insurance.
Principal pays down the amount you borrowed. Interest is what the lender charges to lend it. Taxes are your property taxes, and insurance is your homeowners insurance (plus private mortgage insurance if your down payment was under 20%).
Lenders usually collect the taxes and insurance portions in an escrow account and pay those bills for you, which is why a mortgage payment can rise or fall when your tax bill or insurance premium changes. Lenders also use PITI to gauge affordability: your total PITI relative to your gross monthly income is a core input to your debt-to-income ratio, which helps determine how much home you can finance.