Foreclosure is the process by which a property owner gives up their home to their mortgage lender if they fail to keep up with their repayments.
If a mortgage-holder defaults on their mortgage payments for long enough a foreclosure order can be made by a court.
This allows the mortgage lender to enter and take over the mortgage-holder's property, as its ownership reverts to the lender.
After foreclosure, even if the mortgage lender sells the property for more than the value of the loan the balance will not be returned. Generally a court will grant a possession order rather than a foreclosure order if they can.