This checks if the Jenkins version is older than what's required by the plugin meta data or Manifest. Something to note is that the requiredCore is documented in the metadata found in the update center, but this field does not exist in the plugin manifest. The plugin manifest has a "Jenkins-Version" attribute but from what I could see it looked like this did not always hold the same value as the requiredCore, and seemed to typically have a value much lower than the requiredCore value.
Something else to note is that this does only checks the lower bounds of a plugin's compatibility with a Jenkins version, not the upper bounds.
https://github.com/jenkinsci/bom looks to try to solve the issue with documenting upper bound requirements and can perhaps be incorporated into the tool in the future.