Aperture is a variable size opening in the lens, through which light enters the camera.
Aperture numbers indicate the amount the aperture blades are closing. This is sometimes confusing for people since a small aperture number means a larger aperture opening.
Aperture numbers are measured in stops, and are as follows: f/1.0 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64
The smaller numbers, f/1.0 through f/2.8 tells us the blades are barely closed. These apertures are generally what you want to use for portraits.
A medium number, such as f/8 through f16, has the blades closed down a bit. This is usually the sharpest range of a lens for capturing an entire scene like landscapes.
The larger numbers; f/32 through f/64, tells us the blades are nearly completely closed. The only use for this is to have a longer shutter speed. This could be used for shooting waterfalls, or any shot to show motion blur.
This post contains affiliate links, which means I receive a small monetary reimbursement if you make a purchase using this link.