It is annual disposable income, which is effectively after-tax income ("disposable income" and "after tax income" have slightly different definitions but they are close).

So if the average Canadian household disposable income is roughly $60,000/year and the average household debt is $100,800 then you get a debt to income ratio of 1.68%.

Note that the debt-to-income ratio is a pretty meaningless measure from an individual perspective; one person might have a debt-to-income ratio in excess of 500% and be doing just fine while another might have a debt-to-income ratio of only 50% and be on the verge of bankruptcy. This measure only has some vague sort of meaning at the macro level.