How to interpret this gauge requirement?
Usually, the gauge is given in both stitches and rows, for example 
"Gauge: 10 sts/15 rows = 4 inches in stockinette (regular knit) stitch."  

You rightfully care about gauge - and typically should always check it - to insure that your final product will have the correct dimensions. 

The three elements of how to adjust your gauge are: 
(1) needle size
(2) yarn weight, and 
(3) tension.

You can adust any of the above to get the correct gauge.  Usually, the variable you will modify is needle size, since how you knit is just how you knit (the tension of your knitting is characteristic of how you knit).  

The weight of the yarn is not usually something you want to change since you have carefully picked out your yarn in advance (and are making your gauge swatch from it!).

Since your pattern does not stipulate the number of rows in the gauge, I recommend just checking the number of stitches in the row - the horizontal dimension, and adust to obtain the correct width. (note: As one commentator suggested, a scarf is one instance where the length may not matter as much as it would with a structured item like a sweater.)

Here is a more detailed, but simple, explanation and a good reference for checking gauge: follow:http://sheepandstitch.com/what-is-knitting-gauge/

Good luck and happy knitting!