Similar to the Foreclosure. It means that the Borrower (Mortgagor) has stopped paying mortgage to the Lender and the Lender is allowed to recoup the funds by selling the property.
In the Power of Sale the Borrower (Mortgagor) remains on title until the property is sold. The Borrower (Mortgagor) is entitled to any extra money, after deducting debt repayment and fees, from the sale. The Mortgagor is responsible for any losses incurred from the sale.
In Ontario, the Mortgage Lender is able to evict the property occupants and sell the property.
Power of Sales requires less involvement from the Court System than the Foreclosure do.