The weight of yarn refers to the diameter of the yarn, which determines what size knitting needles or crochet hook will be required to obtain the right gauge for that particular weight of yarn. Knitting and crochet projects usually specify the weight of the yarn and the size of the needle or hook in reference to the project's gauge.
If you are struggling with what weight yarn to use for a project, anyone in the Yarned and Dangerous community will be happy to help you find the perfect yarn for your project.