Your camera has an inbuilt light meter to read the amount of light in a scene when you are taking a photo.
Your camera’s light meter ‘sees’ only tones and senses whether the scene is bright, dark or somewhere in between (a mid tone). A mid tone looks something like this, neither dark nor bright.
If your camera is in one of the auto exposure modes, it will use this light meter to read the amount of light and then choose the aperture, ISO and shutter speed combination that will achieve what your camera considers to be a correct exposure by letting an amount of light hit the sensor which gives you a photo that had a mid tone overall.
Your camera always tries to produce photos with a mid tone overall. If you point your camera at a dark subject your camera’s light meter will read this as dark and will try to create an exposure that makes the dark object appear to be a mid tone. If you pint your camera at something which is a mid tone, it will expose this correctly.
If your camera is in manual mode it will read the amount of light and indicate the light level to you, more on that later.