DTI or Debt To Income ratio is one of the numbers banks use to determine your qualifications for a mortgage loan. It basically takes the amount of your monthly payments and divides it by your monthly income. The payments they consider are credit card payments, car payments, student loans, alimony, installment loans and child support. Basically if you need to make a payment every month it will be counted as part of your debt.
Bankers use the DTI to gauge how well you can afford to make the mortgage payments should they lend you the money. As a general rule your DTI cannot exceed 36%. Loans where the DTI is higher than 36% will generally be charged more interest or be denied.
When you go to the bank, be prepared. Have two years of your W2s to verify income. Also bring your monthly bills summarized on a spreadsheet. If you really want to score points also compute your DTI for the lending officer.