Aperture - Photography glossary

An aperture is a hole. The larger the aperture, the larger the hole. Simple. 

Within photography, an aperture is how big the hole in the lens can go. The bigger/wider the aperture, the more light the lens lets in. Apertures are measured in f numbers or f stops. The lower the number the wider the aperture. f1.4 is a wider aperture than f8, for example.

A by-product of a wide aperture is a shallow depth of field.