These three were taken within the same unlit room. Granted, the natural lighting in this room is pretty good. There are three large windows along the outdoor wall. In the first two, my portrait has been backlit, with my back to the brightest lighting source rather than my face, which is our subject in this case. You can see here that, when focused on the nose, or the subject, the camera automatically exposes for what you're shooting, and the highlights in the background spill around your subject. In the second image, when focused on the lighting source, you can see that less detail is available on your subject and that I am darker. The extra ISO or exposure work that the camera has to perform causes it to render a more noisy image in terms of quality as well.
Let's turn me around. Now I'm facing the windows, or the main lighting source. I point on my eyes or bridge of my nose (in portraits, that is where you want to focus for best detail), and now the camera doesn't have to work so hard so your image quality isn't noisy. The subject is the main thing that's lit, and because I'm facing the window there's a catch light in my eyes. The catch light livens the face(s) of whoever you're photographing.
One more note on this: when taking portraits, there's a nifty portrait feature often built in to smart phone cameras. Go ahead and use that. In the android device I use, it gives me two images in the end--one with the background more blurred (the strength of which can be altered in editing afterward), and another with the original, sharp background.