A sweep account is a bank account that has it balance 'swept' from it on a regular basis.
For example, a cash intensive business, such as a grocery chain, will deposit large amounts of cash every day, which will then be swept to the head office account every night.
Another way a sweep might be used is for a number of subsidiary companies would have their balances swept every night to minimize the effect of any individual overdrafts and increase the earning power of their overnight funds. However, they may have their balances swept back to them the next morning.
Sweep systems can also be set up so that balance are only swept periodically, say once per week, or when the account reaches a certain balance.