I'll give this a shot.
Welcome to photography.
Google is your new best friend. There are also many apps in itunes that can teach you how to capture light through photography. I have downloaded and bought many.
Basically, there are three variables to capturing light; shutter speed, aperture size through the lens & iso in the camera.
ISO is a number, that was established for film, that indicates the sensitivity to light of that medium; then film, now digital sensors. The lower the number (ie ISO 100) the less chance of noise being added to the captured image. Capturing perfect images @ ISO 3200 was unheard of just a year ago. ISO can usually be set to separately to either manual-choice or camera-automatic within a user-defined range.
M mode is manual. You manually adjust everything. There is no reset.
Av mode: you choose the f/ value, camera does the rest.
Aperture priority, often abbreviated A or Av (for Aperture value) on a camera mode dial, is a setting on some cameras that allows the user to choose a specific aperture value while the camera selects a shutter speed to match. The camera will ensure proper exposure. This is different from manual mode, where the user must decide both values, shutter priority where the user picks a shutter speed with the camera selecting the aperture to match, or program mode where the camera selects both.
The main purpose of using aperture-priority mode is to control the depth of field. Aperture priority is useful in landscape photography, where a narrow aperture is necessary if objects in foreground, middle distance, and background are all to be rendered crisply, while shutter speed is often immaterial. It also finds use in portrait photography, where a wide aperture is desired to throw the background out of focus and make it less distracting.
Another common use of aperture priority mode is to suggest how the camera should determine a shutter speed, without risking a poor exposure. In landscape photography a user would select a small aperture when photographing a waterfall, hoping to allow the water to blur through the frame. When shooting a portrait in dim lighting, the photographer might choose to open the lens to its maximum aperture in hopes of getting enough light for a good exposure.