Stub ruleset is a minimal ruleset with which freeciv can be started. It's provided as a base for developing custom rulesets on.
It's currently maintained in main freeciv repo, but as of freeciv-3.0 it's not included in the freeciv distribution. Instead I've now made freeciv-3.0 version available via the modpack installer, in the default modpack list shown when the tool is launched.