In general, the primary cables or pipes that carry a service such as electricity or water. “Mains” may be distinguished from the connections at each end user’s location: electrical mains usually transmit power at a higher voltage, which is then reduced at the connection point.
The British apply the term two ways. It often refers to the service supplied through cables, such as, “My laptop is running on mains electricity”. A second, more audio-centric use refers to the output of a mixing console that is sent to the primary audience loudspeakers, which are sometimes referred to as “mains”, as opposed to monitors or other speakers.