A certified check is a regular personal or business check drawn on the payer's personal or business checking account and the availability of funds is certified by the bank at which the payer holds the account.
You write a check, take it to the bank teller and request that it be certified. The teller verifies that you have enough money in your checking account, sets the amount (plus the certification fee) aside from your account into the bank's certified checks account, then marks the check with a bank stamp indicating the bank has certified the check, then an officer of the bank signs it. The bank then becomes obligated for the funds, not you.
Banks are not required to provide the service of check certification. In that case, request a cashier's check or certified money order.