A secured overdraft is a revolving source of funds that’s backed by cash or cash equivalents, such as a cashable GIC. Since you're using valuable assets to back up your overdraft, you may qualify for a higher overdraft limit at a lower interest rate.
An unsecured overdraft is a revolving source of funds that isn't backed by cash or cash equivalents. Since you're not using valuable assets to back up your overdraft, your interest rate may be higher and your overdraft limit may be lower.