To backlight an image simply means that your light source is coming from behind your subject. You can use backlights in creative ways. One way is to use the sun as your light source. Hudson is my model here. The sun is behind him and to the left, but also is high in the sky, so I manage to have some light on his face. This effect enhances the yellow in his brindle coloring.
Another effect you could create with backlighting would be a silhoutte, or partial silouette. The horse here is mostly in siloutte, but you can see the breeze blowing his mane and tail and the bright sun making his tail glow red with warmth.