Sometimes known as a drawdown facility, a loan which enables you to take out further advances with very little formality. It is usually an option made available with so-called flexible mortgages, allowing you to 'draw' funds out of your mortgage account at any time during the mortgage term, providing you don't increase the size of your borrowing over the previous mortgage advance. This is an option that can also be applied to self-build mortgages.
As a borrower it means you have access to borrowings in the future without having to resort to a remortgage or filling out reams of paperwork. You have the rassurnace that you have the facility to access more money in the future if you need it, perhaps for home improvements.