Plain English Meaning
It is a single switch with two halves. One half turns on the battery, the other half turns on the alternator. You can flip both at once, or use just one side if you need to.
Full Definition
A two-sided rocker switch installed in many light aircraft that combines the master battery switch and the alternator switch into a single control. One half (commonly labeled BAT) connects the battery to the aircraft's electrical bus; the other half (commonly labeled ALT) brings the alternator online to charge the battery and supply electrical loads in flight. The two halves are typically hinged together so both can be turned on with one motion, but each side can also be operated independently when needed.
Context Anchor
Seen on the cockpit switch panel during , engine , electrical checks, and electrical problem .