If you make less than a 20% down payment, you have a high-ratio mortgage. With a high-ratio mortgage your lender will need mortgage loan insurance. Mortgage loan insurance lets you buy a home with a minimum down payment of 5%.
Most Canadian lending institutions require mortgage loan insurance because it protects the lender. If the borrower defaults (fails to pay) on the mortgage, the lender is paid back by the insurer. You pay a premium for mortgage loan insurance.
Your lender will add the mortgage loan insurance premium to your monthly payments, or ask you to pay it in full upon closing.