Wow beautiful line, my guess is, the sun/light represents positivity, so when you're "facing the light" metaphorizes that you are happy, and the shadows or darkness represents negativity, so your negative thoughts and feelings will "fall behind you" get it?
When your back is facing the sun, you become a silhouette, black figure, so you're filled with negativity, sadness.
Direct yourself towards the 'light/sun' and evil dark sad things will not hinder your path.
It simply means be positive. The sun represents light and positive energy, and if you turn towards it, the negative shadows will disappear. You could say it's a metaphor.
it has a double meaning, there's the logical meaning that when you look towards the sun it shines on you and your shadow lies on the ground behind you. there is also another meaning, in our society we see light as good, in religion god is light and the devil is dark, happy things are light and bad things are dark, so when you look towards the sun it is like saying when you look towards the light, when you look towards the good things in life then the bad things will fall behind you. it's like saying when you see the glass is half full you don't focus on the bad things and that makes you happier.
It's easy to equate everything to negative and positive. When I think about turning to face the sun I think of squinting, and a sore head as a consequence. Walking in the shadows is a metaphor that is easily understood. The proverb concerns the shadow, not so much the sun. The sun merely casts the shadow (and makes you squint if you try to defy it), so you could even argue that the sun is a negative force in the proverb. Standing or walking in the shadow(s) represents being unable to get out of the shadow cast by those that have gone before you, or the shadow of the mountain that has stood in it's magnificence for millions of years. The shadow of the mountain could also highlight it's stature in time compared to our own shadow which casts a relatively small stature in time. The mountain is awesome, you are not. the mountain survives, you do not. The shadow of the mountain will be cast long after your dust increases the monumentous size of the mountain. Or yes, you could just relate it to darkness and light. Face the light and the darkness falls behind you. But that's boring and not worthy of discussion.
The meaning to me is that when you look to the sun, your looking at the future and the shadows are the past, and that I don' t look behind you look infront,and also make me feel internally happy, this is my north star....!