The difference between the lightest part of a photo and the darkest part is its dynamic range. It can be thought of as the net amount of light being captured by the camera sensor.
Good Or Bad?
Having a great dynamic range in a photograph does not necessarily make a photograph "good" or "bad". It could just mean that there is a lot of dark parts (shadows), and/or a lot of bright parts (highlights).
Modern camera sensors can capture a ton of information! Simply put, if a sensor has a strong dynamic range, it captures detailed information from both the shadows and highlights. This info/range is recorded as an image and can be processed later (editing).
When it comes time to edit and post-process your images, if you've been shooting RAW and keeping the dynamic range in mind, you can bring back details in the overexposed or underexposed parts of an image. This is fantastic! It allows you to bring out important details and context in your shot.
Example - Bringing back details in the bricks behind your model.
Modern manufacturers of cameras are constantly pushing the limits of dynamic range. Sensors are getting better and better. I haven't even mentioned ISO capability!
If you shoot in manual mode and save to a RAW format, you can document a ton of information, and edit it to your heart's content later.