One thing, clearly seen.
The simplest photograph is also one of the hardest: a single subject, given the whole frame. No competing elements, no busy background, no second focal point. Just one thing, and the attention it deserves.
Painters spent their lives doing this — a single apple, a single face, a single tree. The discipline is in choosing what not to include. When you can clear the frame, the subject becomes everything.
You'll know it worked when
There's nothing else competing for the eye. The viewer looks once, and the subject is unmistakable.
One thing to watch for
A clean background isn't necessarily a blank one. A wall of brick, a stretch of grass, a busy market blurred behind your subject — all of these can feel like a single-subject composition if your subject is clearly the main event. The point is hierarchy, not minimalism.