This patch is quite similar to the onoff~ abstraction for "Turning a signal on or off" with a fade-in or fade-out. Instead of just turning the signal on or off, though, this abstraction routes a signal to one of two outlets (with a fade from one to the other), which you can think of as destination A or destination B, based on whether the number in the right inlet is zero or non-zero.
In the I/O Mappings window within the Audio Status… window you can map the outputs numbered 3-16 to play through outputs 1 or 2 of your available stereo output device (say, Built-In Audio, for example). This allows for testing patches on hardware that has less than the desired number of channels.
The selector~ and gate~ objects serve the same function for audio signals as the switch and gate objects do for Max messages. The selector~ object chooses one signal inlet to pass to its outlet. The gate~ object chooses one outlet out of which to pass its incoming signal.
This patch is an abstraction for panning a sound source among four speakers arranged in a row—or more precisely, in an equidistant arc—in front of the listener. The sound’s virtual location in the stereo field, from extreme left to extreme right, is specified as a control signal value from 0 to 1. The four speakers define the boundaries of the three regions between them, each of which constitutes one third of the total stereo field.