Foreground, middle-ground and background refers to the three basic planes of an image especially but not exclusively when creating a landscape image. You can use the foreground to emphasis and highlight the overall image.
Click on the images to see different types of images that highlight the three planes of an image.
The foreground may add depth, emotion, mood and contrast to an image. Place a subject in the foreground that is very close to the lens while the background extends into the distance. The key decision to be made is to determine what you will use in the foreground to enhance to overall image rather than something that will be a distraction to the main subject of your created image. The foreground should lead the viewer into the overall image. In this image the foreground flower does not necessarily lead the viewers eye to the overall image. Plus we have the distraction of the other flower on the right. Whilst it might be considered a pleasing image the emphasis is more on the close up of the flower rather than a foreground used to enhance the overall image.
One compositional technique is to create your image from a lower angle. Squat, kneel or lie down or lower your tripod to see a different perspective of the scene in front of you. In fact take a number of images from different viewpoints for a more creative experience. Creating images from different perspectives should not be confined to images with a foreground. By considering perspective you will be able to experiment and perhaps find a unique viewpoint.
Another consideration is where you place the horizon line in your image. Many photographers place the place the horizon line in the middle of the frame.
This works well when including reflections in your composition.
Think about the rule of thirds and position the horizon line along one of the horizontal rule of thirds lines.
Placing the horizon on the lower thirds line will emphasise the sky.
Placing the horizon on the upper thirds line will emphasise the foreground.