Wraps per inch (WPI) is a way to measure the thickness of a yarn. This tool is used by spinners, but is also helpful for knitters and crocheters to determine the weight of the yarn and what the approximate gauge will be. This guide also has lines on the back to measure your yarn against, including lace, fingering, sport, DK, worsted, bulky and super bulky.
These yarn guides, made by Sunrise Grove in Washington State, measures about 3 inches tall by 1.5 inches wide and comes with a brass clasp to hang on your knitting bag.
Find a super helpful video on how to use a WPI tool from Very Pink Knits here