Expposure Compensation (“Exp.Comp”, or the “plus/minus” button). What does it do?

Not magic. The only ways to make a picture darker are:

1. Lower ISO
2. Larger F-number
3. Faster shutter

..and the opposite for a brighter picture.  There’s nothing else. Nothing!

So what does it do, this exposure compensation?

• If you are in Aperture Priority mode, it changes the shutter speed (or possibly the ISO, if you are using auto ISO) to a slightly different one than the camera would otherwise use.
• If you are in Shutter Priority mode, it changes the aperture (or possibly the ISO, if you are using auto ISO) to a slightly different one than the camera would otherwise use.
• If you are in program mode, it can change either the aperture or shutter or both (or possibly the ISO, if you are using auto ISO) to a slightly different one than the camera would otherwise use.
• On some cameras, if you are in manual mode, it simply changes the meter reading.

My opinion? Why make things complicated for yourself!

It is best to:

• Use manual exposure mode and not use exposure compensation, which after all is a meaningless concept when you are doing the setting.
• Alternately, when the light changes quickly, you use aperture mode (or shutter mode), combined with exposure compensation for light or dark subjects.
• In all cases you do not use auto ISO!

Simple once you know. Keep things easy for yourself, and learn the three quantities (aperture, shutter, ISO) and how they affect your pictures.

Of course as speedlighters know, when using flash, there is a fourth quantity: flash power!)