anyway, I personally think that the limits are not only on the lens but also the person using the lens. A good photographer can break beyond the limits and produce good photos with the lens.
For eg, some people thinks that f3.5 is limited to bright lights shooting, while a lens with f1.4 is better better for low light environment. However a good photographer who knows how to use external flashes or other lightings properly can overcome that limit.
I'd say the 18-55mm f/3.5-5.6 IS is a very good lens for it's price. newbies should explore the lens to it's max before venturing to other lenses
Aperture is the hole that allows light to enter. So, with a bigger aperture (e.g. f1.2), the hole is bigger. therefore "Brighter"...and also at the same time, it allows more light to enter therefore "Faster"