Aperture determines how much light passes through the lens. Wide apertures let in more light, useful when there is low light. The shutter speed and aperture are directly related. When the aperture is smaller you need a longer shutter speed to let the same light inside. If you dont the picture might be brighter or darker than it should be  Using a wider aperture will isolate a specific subject from the background.

A portrait photographer uses a wider aperture for a shallow depth of field. This focuses on the subject  rather than the background. A landscape photographer uses a smaller aperture so everything can be in focus. This gets everything from the ground and up. Each f-stop is half the size the one before it. A stop is a ‘unit’ of light and exposure value.