Hi and welcome to knitting and to KH!
Gauge is one of the most important things to learn when you start knitting. It'll help you knit mittens (and anything else) that'll fit you. There are wonderful videos on this site including a series under Getting Started
, on the Free Videos tab at the top of the page, that are very helpful. There's a section there on gauge, about 3/4s of the way down the page.
When a pattern says 5.5 sts/inch, it's telling you how to convert the number of sts on the needle into the measurement of your finished knit project. Knit a small piece of about 25sts for about 4 inches and then measure the number of sts over the center 4 inches. Divide by 4 to get sts/inch and that's the gauge you
get with your yarn and needles. If it's not the same as the pattern gauge, you can adjust by changing needle size.
You can get lots of help here with any questions and many suggestions for patterns and ways to knit mittens. Glad you could join us!