Its the Aperture that is denoted by f and it is also known as f stop.

The diffrence between both  is that f/1.4 lets you get more light to your image.

rather f/1.8 lets you get less light in your image.

More the aperture less the light and less the aperture more the light .

and one of the main use of it is the depth of the field.

Suppose you want your background to be blurred at a good rate then use as low apeature as you can.

Hope you will find it helpful.