Once you know WHAT you want to paint, you need to settle on WHY you're painting it. Let's speak in terms of still life only for right now. I arranged this still life, because I wanted to paint this wine bottle with it's colorful label. But the real "big idea" behind this particular arrangement was the way the light traveled across the tabletop and landed squarely on the pears. THAT was the "concept", or the "big idea" of this painting - to try to capture that feeling of the way the light flows.
The importance of having a "concept" can't be overstated. If you don't know why you're painting something, then your painting doesn't have a real purpose. I could certainly have painted this arrangement and accurately portrayed the objects. They may have been believeable. They may have been well done. But, it would have been, yawn, boring!
The concept gives you a clearly defined goal. In this example, it was to try to capture that light.
So, that means that I need to paint everything in such a way as to enhance that idea. What do you see when you first look at this painting? Your eyes go right to the pears and how they almost glow from the strong light on them.
In the real set-up, this distinction was not nearly so evident. But since I had a concept clearly in mind, some objects in the arrangement became subordinant (that bottle almost fades into the shadows on the left) and others (the pears) became the focal point. And I think it worked. You "see" that light!
Next time you try a painting, decide on your "big idea" before you ever start. Then paint everything to communicate that idea. Or, if you're not an artist, then take a look at some paintings, and see if you can determine what the artist's concept was. If you can't, then likely they were simply painting "things" and not communicating an idea.