The path where Solr is installed on the server, relative to the configuration or absolute. Some examples are "../../.." for Solr downloaded from apache.org, "/usr/local/opt/solr" for installations via homebrew on macOS or "/opt/solr" for some linux distributions and for the official Solr docker container. If you use different systems for development, testing and production you can use drupal config overwrites to adjust the value per environment or adjust the generated solrcore.properties per environment or use java virtual machine options (-D) to set the property. Modern Solr installations should set that virtual machine option correctly in their start script by themselves. In this case this field should be left empty!