The Active switch is the one which is the current master of the stack. Priority 15 is the highest priority, and the highest priority will be the master. If the master fails, the next highest priority will be the master.
A stack of switches acts like a single switch, and one switch is the master for the stack, but all the switches have the configuration, so if one switch fails, the stack lives on.
All the switches are actually working. The switches appear as if they were boards in a single chassis. The ports are referred to as `
The advantage of a stack is that you can lose a switch in the stack, and the stack still works. It's just like losing a board in a chassis - you lose those ports, but the chassis still works.