An imprest account is one that holds a fixed amount of money and is replenished after a certain period of time. A good example of an imprest account is a petty cash system that may be replenished on a daily, weekly or monthly basis.
An imprest account limits what can be spent within a fixed time period. For example, if the weekly petty cash limit is $100, it is not possible to spend more. The account is replenished every week depending on how much has been used, always topping it up to $100.
It is also easier to account for spending with an imprest account as the system requires documentation with receipts and invoices for each withdrawal. Since the amount of money is a known quantity, each period's spend is easy to calculate.