A root word has no prefix or suffix — it's the most basic part of a word. The root word at the heart of "conformity," for example, is "form."
In linguistics, a root word holds the most basic meaning of any word. It's what's left after you remove all the affixes — the prefixes like "un-" or "anti-" and suffixes such as "-able" and "-tion." With a word like "lovely," when you take away the suffix "-ly," you're left with the root word "love." Other words, like "schoolhouse" and "armchair," are made up of two root words together.