In order for light to reach the sensor of your camera, it has to pass through an opening in your lens. That opening is called an “aperture” and you have precise control over how large (wide) or small (narrow) that opening is.
The numbers are a way of measuring the opening. The smaller the number (f/2), the wider the opening. The larger the number (f/22), the narrower the opening.
When you go to buy a lens, and you see an “f/#” in it’s title, that number indicates the widest aperture possible with that lens.
Jimages goes into more detail here: https://jimaustin.wordpress.com/2012/12/10/lenses-what-does-1-8-or-1-4-mean/
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