"Faster" is a short-hand way of saying that the lens has a wider aperture, typically expressed with a lower f-stop number. To give some concrete numbers, my Nikon 18-70mm zoom's max aperture ranges from f/3.5 to f/4.5, while my Nikon 50mm prime's max aperture is f/1.8. The prime lens is two to three "stops" brighter than the zoom at max aperture. Stops are factors of two. If you make the lens twice as bright, then you can expose for half the time, thus better freezing motion. Two stops is the difference between 1/8 second and 1/30 second. No matter what kind of image stabilization you have with a 1/8 second exposure, your subject can still move, yielding a blurry picture. With a 1/30 second exposure, you've got a better chance of freezing your subject. If you then pushed your ISO another two stops, you're up to 1/125 second exposure, which will do a pretty good job of freezing motion and without giving up much, if any, image quality.

If you've, in fact, decided to go with Pentax, then the lenses I'd look at buying (with rounded prices given from B&H's web site):

Pentax 50mm f/1.4: $195 (after rebate)
Sigma 30mm f/1.4: $429

This contrasts with their high-end zoom:

Pentax 16-50mm f/2.8: $900 (or $409 for the Sigma lens with similar specs)

You get two stops of extra brightness, relative to the "expensive" standard zoom, or three stops relative to the cheaper, typically bundled with the camera, standard zoom. The only downside of using prime lenses is that you zoom with your feet, although you also have to deal with the lower depth of field. "Fast" lenses like this get the subject is reasonably good focus, but anything just in front or just in behind will be out of focus. To some photographers, this sort of things is a very desirable feature. To others, it's really annoying, because if the autofocus misses the target, you get garbage. Also, if you need anything wider-angle than 30mm, you don't have many choices.

Before shelling out the bucks on all these lenses, you can take the cheap zoom that comes with the camera, and set it at 30mm or 50mm, and see how that perspective works from wherever you're taking your pictures. If you like that field of view, then you may choose to shell out for the prime lens.