Aperture

Aperture (also referred to as f/stop) is how big your lens diaphragm opens through which light passes.

It is calibrated in f/stops and is generally written as numbers such as 1.4, 2, 2.8, 4, 5.6, 8, 11 and 16. When buying lenses I suggest purchasing a 1.8 or 2.0 for a sharp subject and blurry background (the bokeh look). If you want everything in focus - you use a higher f/stop of 8, 11 or even higher.

You want a HIGHER NUMBER f/stop in bright light; 5.6 and up for full sun because it's letting LESS light in.

And a LOWER NUMBER f/stop for low light even as low as 1.2 because it lets MORE light in.

This is the most confusing part of the triangle because the lower the number the bigger the opening and the more light gets let in and the bigger the number the smaller the opening and the less light that gets in.