本 is a tree 木 with a short stroke marking its base: the root. One added line turns 'tree' into 'where the tree comes from' — one of the cleanest examples of an indicative character.
本 grew from root to origin to foundation: 日本 (Japan — 'sun's origin', where the sun rises seen from China), 本子 (notebook), 一本书 (one book — 本 is the measure word for books, with 书 you learned). 本来 means 'originally'. Confucian thought prizes 务本 — attending to the root of things — and the word for 'capital' in economics, 资本, is literally 'resource-root'.
A tree 木 with a line at its base — the ROOT.
日本 'origin of the sun' — Japan's own name for itself (Nihon/Nippon) uses these exact Chinese characters.