So on a standard Ravelry entry there is a Yarn Ideas tab, where you can see what other folks used to make the project.
You may have noticed that on the main pattern details tab and the Yarn Ideas tab are links for yarn in your stash. What that means is Ravelry has looked at yarns you put in your stash in Ravelry and compared it to what yarns other crafters have used to make recommendations on what yarn you already have that might work to make a pattern.
This is a great feature; it not only encourages you to use your stash, but also encourages you to catalogue your stash so you know what you have and are more inclined to use it!
Speaking of which, I have the perfect yarn for this hat sitting in my stash…