Gauge means the number of stitches per inch and rows per inch that result from a specified yarn worked with a specified-size hook. Since everyone crochets differently—some loosely, some tightly, some in-between—the measurements of individual work can vary greatly when using the same-size hook and yarn. It is your responsibility to make sure you achieve the gauge specified in the pattern.
Hook sizes given in instructions are merely guides and should never be used without making a 4-inch-square sample swatch to check gauge. Make the sample gauge swatch using the size hook, and the yarn and stitch specified in the pattern. If you have more stitches per inch than specified, try again using a larger-size hook. If you have fewer stitches per inch than specified, try again using a smaller-size hook. Do not hesitate to change to a larger- or smaller-size hook, if necessary, to achieve gauge.
If you have the correct number of stitches per inch, but cannot achieve the row gauge, adjust the height of your stitches. This means that after inserting the hook to begin a new stitch, draw up a little more yarn if your stitches are not tall enough—this makes the first loop slightly higher; or draw up less yarn if your stitches are too tall. Practice will help you achieve the correct height.