Gross debt service covers the mortgage payment, property tax and heating cost, and sometimes half of condo fees where they apply, all divided by gross household income. Total debt service adds every other recurring debt payment on top of that same housing figure.
A minimum credit score of 600 is required for an insured mortgage regardless of how strong these ratios look, and a lender can apply a stricter internal limit than the guideline minimum depending on the rest of your file.
A mortgage broker can run your actual numbers against both ratios before you start looking at homes. Get your hand-reviewed valuation from Joel Dyck.