"f/2.8" is an indication of the diameter of the aperture of the lens, and is precisely what it says, where "f" is the focal length, the "/" indicates division and the "2.8" is one of a common series of numbers used for this purpose. Frequently, the "/" is missed out, but the meaning is the same. The common series includes f/2, f/2.8, f/4, f/5.6. For a 100mm focal length lens, the diameter of the aperture is

d=50, 35.71, 24, 17.86mm etc.

More importantly, the (light passing) area is pi. d^2 /4. So we get 1964, 982, 491, 245 sq.mm i.e. every step halves the light passing area. The brightness on the film or sensor is the same whatever the focal length, and this is why aperture, expressed in this fashion is important and the standard way of doing things. Lenses not only have Maximum Apertures which indicate how suitable they are for low light usage, but also have variable aperture mechanisms which allow the light passing are to be reduced. Similarly, the shutter can be adjusted to be open for different lengths of time, and these too are frequently set out in a doubling series 1, 1/2, 1/4 etc. with slight rounding variations along the series.

The IS version of the "kit" lens (Note: only 1 "e") has quite a good reputation, better than the earlier version, and will allow quite a range of work. Few lenses work superbly well at maximum aperture, so photographers often choose to work at smaller apertures such as f8 or f11 where faults (aberrations) will be reduced, but the shutter speed may then have to be longer introducing a risk of camera shake. It's all a matter of compromises, and today's cameras are excellent at offering good quality results with fewer compromises than earlier. (Cf film at an ISO (film speed) of 1600, very grainy, versus a DSLR set to 1600 ISO, now fairly free of noise.)