PAN is short for PANORAMA, which loosely means “the entire space from left to right”. Mixers work in stereo (2 separate channels) that correspond to our 2 separate ears. The PAN control lets you position the sound of each mic at a specific place between left and right.
When mixing a live band, you might want one instrument’s sound to be a little left of center, and another one to be all the way to the right.
In broadcasting, we generally want the microphones to be centered (equal in both channels), so the PAN controls seldom get moved from that position.
The one major exception to that is when recording a telephone interview for use in a later broadcast. By panning your mic all the way to the left, and panning the caller all the way to the right, your 2-channel recording will have each voice isolated on its own track. This makes it very easy to edit the sound file, especially those times when both of you were speaking at the same time.
So, pan hard left and hard right when recording an interview. But for all other purposes, set the pan controls to the center.