Exposure Compensation is an absolutely critical feature of the camera to understand.
Exposure Compensation is a way to brighten or darken the image being taken without adjusting any of the properties manually.
You are taking the camera’s measurments (light-metering) and saying “okay, that, but brighter” or “sure, that, but darker”.
The exposure compensation icon is universal. It looks like this:
Exposure compensation doesn’t do anything in manual mode because we aren’t using the camera’s settings. This is why the exposure compensation button is also – while in manual mode – what often serves to toggle which setting your dial adjusts – shutter speed or aperture, in manual mode.