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.
Exposure Compensation Icon by Martin Chapman Fromm from the Noun Project