If you are retired and you own your home, or have a lot of equity in it, you can sell your house to generate money for your retirement (because lots of people buy a house just as an investment for retirement!) Or you can stay in your house and 'borrow' a lot of money against it, to get your equity out. A reverse mortgage is a deal where you borrow money against your house, and you don't pay it back until you sell the house (or your heirs sell it after you die).
You can get the money all at once, or you can take it in weekly or monthly installments, or borrow against a fixed limit as you need it. You can even use it as a 'revolving' plan where you can borrow money, pay it back, borrow it again, etc., so long as you don't pass a limit. You pay interest on the money but it is a very low rate because it's collateralized by the house.