As far as I remember it's how much of that host to use, so that some of it could be held for the OS/outside Rancher.
I'm glad you like our old software but it's unwise to be still using it. Upgrade to 2.x or find something else (if you don't want Kubernetes, look at Portainer).
You're using an entire stack of multiple layers of software that has long been unmaintained by anybody and is surely full of vulnerabilities by now.
Virtually nobody still employed by Rancher/SUSE has ever even seen 1.x because it's been about 4 years since it was actively developed in any meaningful way.