Let’s say that you’re at a viewpoint on the side of a hill and you’re leaning on the railing trying to decide on the composition for the best photo you’ve ever taken in your life. The valley is directly below you. In the bottom of the valley is a river flowing through meadows interspersed with stands of trees, and on the other side of the valley soar snow-capped peaks. A bird rides on the air currents above the valley. To the side of the viewpoint are a few scraggy pine trees. You think you might see some deer or elk grazing near the river, but it’s so far away that it’s hard to tell.
In this scene, there are many subjects to choose from – the mountain peaks, the deer or elk, the trees, the flying bird, etc. At this point, the entire scene is so beautiful that it’s tempting to just say, “I want to capture it all!” But that would mean having multiple subjects. And most pictures are better when they have one primary subject.
You decide to make the river your primary subject. The foreground, then, is the valley (meadows, trees, etc.) between you and the river and the background is everything beyond the river (the meadows, forests, snowy peaks, etc.)
You’ll want to set your equipment so the river will be in focus (the subject should be in focus in almost all pictures). Good. But how do we make the river the subject and let the foreground and background be less visually interesting and therefore not distract from the subject?
This is where you’ll need to move around. You want some foreground and background in your picture. But you also want said background and foreground to help tell your story not detract from it. I’d probably frame the photo so the river draws your eye through the picture. The other elements of the valley are still there, but your eyes wander with the river instead of settling on the mountains, meadows, etc.