The combination of ISO and shutter speed and aperture gives you an exposure value. Changing the ISO or shutter speed or aperture results in one of the others changing in the opposite direction to maintain the same exposure value. For example, if you double the ISO you then half the shutter speed to maintain the exposure value.
You change the exposure compensation when you don't like the exposure value picked by the camera. For example, the camera meter may be confused by all the snow around your skier friends and underexpose the scene because the camera does not know that all that snow is supposed to be white, not middle gray. You then might need to add some +EV to let in more light to get the snow back to white and your friends not underexposed.
There are two ways to change your EV compensation. In manual mode, you just change one of the exposure settings (shutter, aperture, ISO) -- handy for just a few pictures. Is any of the auto modes, you set the EV value to a +/- value and the camera does the calculations -- handy for many shots.
---------+---------+---------+---------+ Joseph K Seattle, WA, USA