Several parts combine into one character.

Pictograph: a lamp stand (王 element below — actually a pillar) with a single flame (丶 dot) on top. The encoded scene: "the wick of an oil lamp" — the central flame around which the household gathers. From this physical center grew the abstract meaning "host / owner / main / principal". Identical across 繁體 / 新字体 / 简体.

主 anchors a wide vocabulary of leadership, ownership, and primacy across CJK languages. The 〜主 suffix names roles ("owner of X / chief of X"): 店主 (shop owner), 君主 (monarch), 領主 (feudal lord). The 主〜 prefix marks "main / primary": 主要 (main), 主題 (theme).

The character also forms one of the most important political/ideological suffixes — 主義 (-ism). 民主主義 (minshu shugi, democracy — "people-rule-ism"), 資本主義 (capitalism), 社会主義 (socialism), 共産主義 (communism). Modern political vocabulary in CJK languages depends on this character.

Mandarin: zhǔ, dipping 3rd tone. 主要 (zhǔyào, main), 主人 (zhǔrén, host / master), 民主 (mínzhǔ, democracy), 主席 (zhǔxí, chairman / president — Mao Zedong's title 毛主席), 主义 (zhǔyì, ideology / -ism), 主义 (zhǔyì, doctrine / -ism), 业主 (yèzhǔ, business owner), 主张 (zhǔzhāng, advocate / position).

Japanese: TWO on-readings. シュ (shu) is dominant — 主人 (shujin, husband or master), 主要 (shuyō, principal), 主題 (shudai, theme), 主婦 (shufu, housewife), 民主主義 (minshu shugi, democracy). ス (su) appears in 法主 (hosshu, head of a Buddhist sect). Kun-readings: ぬし (nushi, owner / proprietor — used for landlords) and おも (omo, main / chief — おもに = "mainly"). The archaic お主 (onushi, "thou") survives in samurai dramas.

Memory aid: an oil lamp wick — the household's central flame becomes the master.