[em500]

More specific, it's the outstanding debt to disposable (i.e., after tax) yearly income.

Even more specific, I it's probably the total outstanding mortgage debt in a country divided by the total disposable household income in the country, including incomes of non-mortage holders and renters. If you think that denominator doesn't make much sense, I largely agree, but it's fairly standard to calculate it that way in macro-economic reports. So I believe the article writer actually misinterpreted that number, writing that it's 150% of homeowners income.