By foreclosing the landlord's property, the bank actually wants to take over the house and sell it at a public auction thereby getting back wheveter money he has offered as loan. Your landlord may have defaulted on the loan as a result of which the property will be foreclosed.
As Adonis said, if a person takes mortgage and is unable to continue making payments it results in defaults. In such situation the lender has the right to foreclose, which means take back the property and sell it to recover his dues.