the root of a plant: the underground part that anchors and feeds it, and by extension the hidden base from which something grows, draws life, or takes its origin
The word names the root of a plant, the part below the surface that holds it steady and draws up water. The New Testament uses it for actual roots: a tree with the axe already laid to its root, a seed that dries up in the sun because it never rooted, a fig tree dead all the way to its roots.
From that plain picture grows a family of figurative uses. A person can have "no root," meaning nothing inside to keep them steady when trouble or persecution comes. A community can have a root that feeds it: in Paul's image of the olive tree, the root is the faith of the patriarchs, and the grafted-in branches live off it rather than the other way around. And a thing can have a root in the sense of a source, as when the love of money is called "a root" from which all kinds of evil grow.
The most striking use is as a title. Drawing on Isaiah's promise of a "root of Jesse," Revelation calls Jesus the "Root of David." Here the word points in two directions at once: he is the source the royal line springs from and the living shoot that grows up out of it. Scripture elsewhere calls the Messiah the "seed" of David (σπέρμα), naming him simply as the descendant; "root" says more, that he is both descendant and source. The Hebrew word behind these passages, שׁרשׁ, carries the same double image.