A forest often has a lot of "undergrowth" - smaller residents of the forest that are happy to get what little bit of light that happens to filter through the thick canopy of leaves overhead.
Usually, the "undergrowth" refers to plant life that has adapted to survive on the small amount of sunlight they can soak up.  Ferns, moss, and such.
The other dwellers down there, the smaller trees that just aren't tall enough to get much sunlight, have it much rougher.  They need the sunlight - but they don't always get as much as they need.
It's rough out there.  I won't sugarcoat it - a lot of 'em just don't make it.
Anyway, here's a close-up image of some leaves from one of those smaller guys, partially lit by filtered sunlight cutting through the otherwise shady environment:
Columns of sunlight tend to shoot down through the forest canopy for brief moments, and then go away just as quickly.  It depends on the position of the sun in the sky, the swaying of the canopy leaves in the wind, and even the movement of the clouds. 
When a moment comes along in which rays of sunlight travel over 90 million miles to Earth, break through the clouds and the forest roof, and illuminate some leaves that are otherwise in the shade, triggering the photosynthesis process in order to feed some poor little tree for a few seconds, well... it's nice to capture that moment in a photograph.
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