Phantom Power

Condenser mics contain active electronics and need power to work at all. Rather than take a battery, they draw direct current sent up the same balanced XLR cable that carries the signal back, labelled 48V or +48V on the device and usually a button per channel. Dynamic mics such as the SM7B do not need it and are unharmed by it on any modern interface, so leaving the switch on is normally fine. The exception is ribbon mics and some vintage gear, where phantom power through a miswired cable can do real damage. If a condenser is silent, check this first.