Several parts combine into one character.

怖 is a phonetic-semantic compound: 忄 (the upright form of "heart" used in compounds, the heart radical) + 布 (bù, "cloth / spread out" — providing the sound and a hint of meaning). The composite metaphor is wonderfully physical: the heart trembling and spreading like cloth disturbed by wind = fear. 怖 pairs with 恐 (kǒng, also "fear / terror") to form one of the most common Sino-CJK compounds of dread.

Korean reading "po." Korean rarely uses 怖 alone — almost all "fear" usage is funneled through the single high-frequency compound 恐怖 (gongpo, fear / horror — used for everything from horror movies to existential dread to phobia diagnoses): 恐怖映畫 (gongpo yeonghwa, horror film), 恐怖症 (gongpojeung, phobia), 恐怖政治 (gongpo jeongchi, terror politics / reign of terror). The single character 怖 by itself appears only in highly formal compounds like 怖懼 (poku, fear and dread).

Mandarin bù, 4th tone. 恐怖 (kǒngbù, terror), 恐怖片 (kǒngbùpiàn, "terror film" = horror movie — what Chinese audiences call slasher / horror genre), 恐怖主义 (kǒngbù zhǔyì, "terror-ism" = terrorism), 恐怖分子 (kǒngbù fènzǐ, terrorist). The Mandarin pattern 恐怖 + noun is highly productive in modern political and entertainment vocabulary.

Japanese on-reading フ (fu) — 恐怖 (kyōfu, terror), 恐怖症 (kyōfushō, phobia — appears as suffix in psychological terms: 高所恐怖症 kōsho kyōfushō = acrophobia / fear of heights). But where Japanese diverges from Korean and Chinese is the kun-reading こわい (kowai, "scary / frightening") — one of the very first adjectives every Japanese learner masters. 怖い (kowai) appears in everyday Japanese constantly: 怖い夢 (kowai yume, scary dream), 怖いよ! (kowai yo, "I'm scared!"), お化け怖い (obake kowai, "ghosts are scary"). Note the homophone caution: こわい spelled as 強い means "strong" in some Japanese contexts (archaic / dialectal). The verb 怖がる (kowagaru, "to be afraid / show fear") is the active form. Japanese makes 怖 part of daily emotional vocabulary in a way Korean and Chinese reserve only for elevated compounds.

Memory aid: a heart (忄) trembling like spread cloth (布) — the visible, physical disturbance of fear.