root: the underground part of a plant, and by extension the hidden base or source from which something grows, holds firm, or lasts
At its core this word names the root of a plant or tree, the part hidden under the ground that anchors it and draws up its life. The Hebrew writers use it constantly for real roots, but even more often for people and nations pictured as trees: when the root is well watered the tree thrives, and when the root dries up or is torn out the whole plant dies. Because so much depends on the root, it becomes a natural picture of whatever a thing secretly rests on.
From that literal base the word reaches outward in a few directions that English speakers will find familiar, because English uses "root" in almost the same ways. It can name the source or origin of something, the hidden beginning a matter grows from, whether that is a debated question or a moral poison. It can name firm footing, being so deeply rooted that one cannot be shaken. And it can stand for the living stock of a family line, the base that survives and sends up a new shoot, so that the "root" of a person can even mean his descendant.
A few times the word simply means the lowest or deepest part of something, the very bottom: the base of the mountains, the depths of the sea, the underside of the feet. Here the plant image has faded almost entirely and only the idea of "the part farthest down" remains.
One image ties the whole range together: what is out of sight but decisive. A plant lives or dies by what is underground, a matter is settled by its hidden source, and a family survives through the stock that remains. That is why the same word can move so easily from a literal root to the root of a problem to the root of a royal line.