When we take a photo, we tend to concentrate on the main subject and not think too much about the surroundings or the background. Then, later on we look at the picture and realise we placed Uncle George right in front of a lampost and it looks like it's growing out of his head! Similarly, when taking a landscape photo, we might be concentrating on getting a good picture of a mountain but not notice the garbage bin in the foreground. This hint is all about avoiding these situations.
Some of the best images are the simplest ones, with nothing to distract from the subject. A painter has the option of leaving things out from a scene by simply not painting them. In photography, the camera captures everything, so it's necessary to make the camera see just what you want and leave out the rest.
There are various ways to achieve this. If you're taking a photo of a person, you can put them in front of a plain background or set the camera to a large aperture (i.e. a small f number) so that the image will be in focus on the person's face but the background will be blurred. Like this:
In this photo I've used a plain background (a grassy bank) and made it even less of a distraction by using a large aperture to throw it out of focus. Your camera may have a portrait mode which will attempt to do this for you automatically. Or you can use aperture priority and set a wide aperture (check your camera manual) . If you have a zoom lens, use a long focal length (zoomed in) as this will also help keep the background out of focus. Be careful though not to make the aperture too wide as you might find only parts of the subject's face are in focus. In general, portraits need to have the eyes in focus at least.
For landscapes you obviously can't move the subject, so you have to move the camera to a good location. When framing the picture, look for any distracting objects and move around or zoom in a bit to try and avoid them. For this one, I wanted to capture a still misty morning on Dove Lake near Cradle Mountain:
If you have a tripod, you can experiment with longer exposures. This can have a very nice effect on water or cloud, smoothing it out and simplifying the image. Sometimes the bright or contrasting colours in a scene can be a distraction. Try converting the image to black and white.
Sometimes foreground objects are great for providing context or for leading the eye into the scene, in which case you want to keep them in but have them in the right place. That's a topic for another time...