Two more notes to add--a larger aperture is represented with a smaller f-number. Just as 1/8 is smaller than 1/2, f/8 is a smaller aperture (and creates a larger depth of field) than f/2.
Secondly, if you're stuck with the 18-55 f/3.5-5.6 kit lens, you're limited on maximum aperture: to f/3.5 @18mm (zoomed all the way out), and f/5.6 @55mm (zoomed all the way in). If you can afford a 50mm f/1.8 lens, that's usually the cheapest way to acquire a "fast" lens--one with a max. aperture of f/2.8 or wider.
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