What specifically does "against proof of a rental contract" mean? Rental contracts, at least apartment ones here in the states, often require some up front cash

What specifically does "against proof of a rental contract" mean? Rental contracts, at least apartment ones here in the states, often require some up front cash. Is this not the case in Finland? If it is the case, is the government provided money given to the render before or after the contract? If given before, how do they prevent people from just stealing it? If given after, how do people without money provide the up-front cash? These and similar questions are some common reasons behind using vouchers/stamps instead of cash.