That might be referring to Yarn's "offline mirror" feature. When enabled, Yarn will cache package tarballs in the designated folder so that you can commit them to the repo. When someone else clones the repo and runs `yarn`, it will look in the offline mirror folder first, and assuming it finds packages matching the lockfile, use those.

This takes up _far_ less space than trying to commit your `node_modules` folder, and also works better cross-platform.

I wrote a blog post about setting up an offline mirror cache a couple years ago:

https://blog.isquaredsoftware.com/2017/07/practical-redux-pa...

Used it on my last couple projects at work, and it worked out quite well for us.