Aperture can be a difficult subject to get your head around, but once you understand the basics, the knowledge will help you a lot in exposing your images correctly, and getting the right depth of field, for the photograph in question.
The maximum aperture available to you might vary depending on the type of lens used, but here we'll cover the whole aperture spectrum to help you understand the term.
Aperture is the name given to the lens diaphragm, which allows light into the sensor of the camera. The wider the hole in the diaphragm, the more light is allowed into the sensor. The size of the opening is calibrated in f stops that are written as numbers: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16 and 22.
The smaller the number of the f stop, the larger the aperture opening. A small aperture number (large opening) will create a shallower depth of field, allowing the background to blur. This is ideal for when there are distracting objects in the background, or you want to separate your subject from the background. Because there is a larger opening, you'll be able to use a faster shutter speed for your image.
The larger the f stop, the smaller the aperture opening, and the more front to back sharpness you'll be able to achieve. A larger aperture number is ideal for shooting landscapes and far away objects, where having everything in focus is necessary. Having a higher aperture number means the hole in the diaphragm will be smaller, and this will, in turn, mean that a longer shutter speed will be necessary.
All Nikon NIKKOR lenses list their maximum aperture on the lens barrel next to the focal length of the lens. To get used to using different apertures and the effect they have on your images, set your camera to aperture priority mode. This will allow your to change the aperture whilst the camera will automatically set the appropriate shutter speed.