Gauge is the number of stitches per inch that you get with a particular size of needles and a particular thickness of yarn.
To measure your gauge, lay out your swatch and measure it. Now, divide the number of stitches you have by the width of your swatch in inches. That will tell you how many stitches you get per inch.
If it doesn't come out the same as the pattern's gauge, you can either knit tighter or looser or change needles sizes. HTH