Making a Gauge swatch before knitting a project is an important step if you want the sizing of your project to be accurate.  Gauge is mostly important for fitted items like hats and sweaters where fit matters and less important for items like scarves and blankets.
A Gauge swatch will help you determine if you are knitting too tight, too loose, or just right! 

 

Measuring Gauge in Garter Stitch
Reference for the below kits:
  

Measuring Gauge in Stockinette Stitch
Reference for the below kits:
 

Measure Gauge in Fisherman's Rib Stitch
Reference for the below kit: