Aperture Priority mode is considered a "semi manual mode" because, while you choose the aperture, the camera will choose the shutter speed that matches it to give you proper exposure. -It's really quite easy to use...
The smaller the number you choose for the aperture, the more shallow your depth of field will be. That means small numbers = more background and foreground blur. Large numbers will give less blur.
**NOTE:** Distance from subject to camera, distance from subject to foreground / background, and the focal length of your lens (measured in mm, like a "50mm lens") will also effect the depth of field.