The gauge is to ensure that you knit (or crochet) with the same tension as the pattern designer. This will make your project come out to the size that the pattern specifies.
If you do a swatch and it comes out bigger than the gauge, you can either switch to smaller needles (hook), or you can try to knit (crochet) tighter.
If it comes out smaller, then you can either switch to larger needles (hook), or you can try to knit looser.
Hope this helps!