Aperture is the opening inside the lens. It affects exposure, depth of field and the visual separation between subject and background.
Wide apertures
Values such as f/1.4, f/1.8 and f/2.8 let in more light and create a shallow depth of field. They are useful for portraits, low-light scenes and images where the subject should stand out against a soft background.
Smaller apertures
Values such as f/8 or f/11 increase depth of field and are often useful for landscapes, architecture and product photography. Avoid closing the lens more than necessary, because very small apertures can reduce fine detail through diffraction.
Choose for the image
Do not select an aperture only for exposure. First decide how much of the scene needs to be sharp and how strongly you want to separate the subject. Then combine aperture with shutter speed and ISO to make the exposure work.
