Visual Priorityis how a piece of art tells your eye what to do. Naturally, important things (foreground elements) should be at the top of the list. That way, the second you look at something, your eye immediately recognizes "I should be looking here".
There are a few ways to do this.
This is a striped box on a striped background.
1. They sort of run together a bit. It's obvious why- they're the same colors and have nearly identical features. If it wasn't for the two horizontal lines, the box would be nearly invisible. This has very little variation in visual priority.
2.
Contrast- Look at a picture of a landscape. The further the land goes back, the more is dissipates into the atmosphere. That's about all there is to it. High contrast things stand out, and standing out is exactly what brings your eye to that thing.
3.
Hue- This is basically the same principle as contrast, but this is not the one you want to rely on. It's still good to use, though.
4.
Value- Also the same principle as contrast.
5.
Shape- This can either be achieved externally (the silhouette, e.g. that circle) or internally (the pattern in the box). It's probably where your piece is the weakest. It has a
lot of vertical lines, and with the background being at the same priority in most other aspects, they're very hard to separate.
6.
Level of Detail- And back to the same principle as contrast. This is a big one for games. With a busy background, foreground elements easily get lost.
I'd go into more detail if I had the time but I don't. EDIT: That sounded really douchey, I hope it didn't come across too bad.