本 takes the tree you already know, 木, and marks its base with a single stroke — the scribes pointing straight at the roots to say "here is where it starts." A root is the beginning of everything a tree becomes, so 本 grew from "root" into "origin" and "foundation," the sense it still carries in words like 根本 (gēnběn, the very root of a matter).
A piece of culture
The root of a book 本 also became the word for a bound volume of writing — a "root text," the base copy everything else was measured against — which is why 本子 means notebook, 课本 means textbook, and 本 is still the measure word for books today, as in 一本书 (yī běn shū, "one book").
Use it in a real word
本子
běnzi · bun2 zi2
notebook — literally "root-child"; you already know 子