A gauge is the measurement of the number of stitches per inch. The gauge given in a pattern is simply the number of stitches the designer achieved per inch. In order for your knitting to match the dimensions of the pattern, you need to knit with the same number of stitches per inch as the designer. A test swatch is knit in the required stitch pattern and suggested needle size. The swatch is then measured to determine if it matches the same number of stitches per inch as the pattern. Because each knitter has their own unique tension, it is not unusual to try several needle sizes before achieving the required gauge. This dreaded bit of experimentation is crucial to proper sizing.
Every knitter wants to jump right into their new project and skip the swatch/gauge step. Avoid this temptation and spend the valuable time calculating your gauge. You will be much happier wearing a perfectly sized garment. This advice comes from the knitter who made a sweater without a gauge for herself and instead it fit her husband. (Unfortunately, I couldn’t get him to wear the pink cable sweater!)
Check out this video and learn to correcting calculate your gauge for your next project.