A hem gauge is exactly what you think it is. It is a metal "ruler" used for measuring and ironing the hem of a garment before sewing. It can be a challenge to get a nice even hem. You could sew a basting stitch around the area to be hemmed then iron the fabric folding at the basting stitch. Or, you could just use a hem gauge and eliminate the need to sew a basting stitch then pull out said basting stitch. Because the gauge is metal you can safely iron over it. One word of caution, the gauge is metal so if you iron right on it be careful it will be hot (I have learned this lesson the hard way more than once, you're welcome).
There are a variety of measurements in both inches and centimeters. You can use a hem gauge for a straight hem. Or...
...a curved hem.
I use mine all the time. It is large so doing the hem on a small sleeve or pants leg can be difficult. It's still a handy tool to have in your arsenal.