A service charge is an annual fee payable by a leaseholder to a management company or agent for the maintenance of the building and common areas. This is particularly common in flats, but not exclusively.
It's worth checking whether the property you are purchasing carries a service charge and if so, how much this will be. It's worth being aware the fee can vary based on the costs incurred.
Speak with your conveyancer regarding the terms of the lease regarding the service charge if you have any additional questions.