Scale in general tends to work like this:
1/how many of the doll you'd have to line up head-to-feet to be the size of a human
Basically it's to indicate how much a real person would have to be shrunk down to be the same height as the doll, or how much you'd have to multiply the doll's height by to get their 'height in real people size', tho often doll proportions are all over the place and not all that anatomically accurate.
A 1/6 doll like monster high or barbie is such because you'd have to multiply every inch or centimeter of their height by 6 to make one roughly as tall as a real person. They are 1/6 the height of people. Same goes for 1/12 (common dollhouse scale), 1/3, 1/4, etc. Naturally, with kid-sized dolls like Barbie's Kelly, they're still 1/6, as the scale is comparing them to a child instead of an adult.
Or, to put it another way, every 2 inches on a Barbie or Monster High would be 12 inches or a whole foot to a real person, because 1/6=2/12.
This is also useful to know for making furniture and accessories, though not so much on clothes because of weird doll proportions. For example, if you want to make a doll version of your desk or computer, you can measure it. Then, if you divide the measurements by 6, you'll know what size to make a miniature version for a 1/6 doll. I recommend centimeters as they're smaller and thus give easier results when scaling down.
1/6 is also commonly referred to as playscale because the majority of cheap vinyl fashion dolls marketed for kids to play with are in this scale. (Not meant to be derogatory towards cheap vinyl - that's my preferred kind of dolls! XD)
I hope this helps.