What do linking root domains mean?
When a site links to another one or more than one time in Search Engine Optimization, site A is said to be a linking root domain; the plural, linking root domains is the total number of separate links that link once or more than once to a given site. If a domain links to a site more than once it is still counted as just one linking root domain.
The popularity of a site can be studied by looking at the linking root domains, but it is not as simple as looking at the number of sites that your content is linked to. Content that is linked to two authority sites will rank far higher up the search engine results than content that it linked to a dozen or more mediocre blogs. Even that is too simple however as links from individual pages are often more powerful than a link from the root domain itself. And consider too that linking root domains are only one of thousands of factors used by search engines when they generate SERPs.