What is it?
How do I use it on my camera?
Depending on the camera make and model you have, by using the A or Av (Aperture Priority) mode on your camera dial or menu, you can select the F number that you want the photo to be taken at, the camera will set the shutter speed to maintain the correct exposure. You would use the main scroll or menu to change the F number.
What does it do to my pictures?
Simply, the smaller the F number you use the less will be in focus in your pictures. This will make the object in focus stand out more. However, there are times that you want lots of the picture in focus, like a landscape or a group of people. To get all of the picture in focus use a larger number.
Get your camera, set it to Aperture Priority mode, start at the smallest number you can and then take a series of pictures of the same thing changing the F number. Note how your pictures are changing. What else is changing when taking these pictures?