It can add to the composition and help set the mood of a picture, or it can detract from the subject if it is cluttered. Watch out for backgrounds that are more compelling than the subject. Cluttered, distracting backgrounds often spoil otherwise good pictures. Before you snap the shutter, stop for a minute and look at the background. Beware of an uncovered trellis or the side of a shingled house when you take informal portraits or group shots, because prominent horizontal or vertical lines detract from your subject.
Foliage makes a better background.
A blue sky is an excellent background, particularly in color pictures.