In XAML, the Source property of the Image control is the path of the image file that you want to display. When you access the same from code, it's a BitmapImage.
The Stretch property of an image describes how it should be stretched to fill the destination. For the first image, that we set as Stretch= "None", it preserves the original size of the image. When you set it as Fill, for the second image in the example, the content is resized to fill the destination dimensions without preserving its aspect ratio.
For the third and fourth image, setting it to Uniform and UniformToFill, respectively, set its content resized to fit in the destination dimensions while preserving its native aspect ratio. But for the fourth case, if the aspect ratio of the destination image differs from the source, the source content is clipped to fit in the destination dimensions.