What is Root for Sitecore Crawler for Index
The crawler root item specified for the index is indexed during index update operations.
For example, given the following configuration for given index, items that are under of the /sitecore/content tree may appear in the index:
<crawler type="Sitecore.ContentSearch.SitecoreItemCrawler, Sitecore.ContentSearch">
If you spy the Sitecore.ContentSearch.SitecoreItemCrawler Class you will find that this string value is further used to set RootItem.