The object of the picture is dramatically darkened by an overexposed background such as a window with bright sunlight coming through.
It's all about how you meter the scene.
Firstly, there is always a range involved with photography. We call it the dynamic range. Basically, this means that we can't expose for very bright areas and very dark areas at the same time. If you set the exposure for the darker areas, the bright areas (highlights) will be blown out (white) and have no detail. If you set your exposure for the bright areas, then the dark areas will be black shadows.
Now, the problem is that the camera doesn't know what your subject is. All it knows are the light levels that are in the scene. So if you have a bight window in the scene, the camera's meter might think that it's an overall bright scene, and it will expose for that....which leaves your subject underexposed.
What you can do, is ensure that the camera is not taking the window into account when deciding the exposure value. You can do this by taking a reading off of the person, rather than the whole scene. Check your manual for how to lock the exposure, it's probably as easy as half pressing the shutter release button.
This will give you a well exposed subject and a really bright window...which is probably better than a somewhat bright window and a shadowy subject.
Now, if you don't want the window to be blown out, then you can try to even out the exposure. One way to do this, as mentioned, would be to use the flash to light up the subject. It may take some tinkering to get the balance between flash and ambient just right, so practice when you can.