Depth of field.

Aperture is the hole that controls the amount of light that is allowed through the lens. It is represented in the camera as the 'f number'. This effects shots by either allowing more light into the image or less. When there is a shallow depth of field(small number, Big hole) People most commonly use this for close ups, portraits and focal points.

When there is a wider depth of field (small hole, bigger number) people most commonly use this for landscapes and to allow less light. Such as the images below.