Several parts combine into one character.

Compound ideograph: 亻 (person) + 言 (speech / words) = trust / belief. The encoded ethic — "a person's word is the person" — is one of the most concentrated moral statements in the script. Confucian commentaries treat 信 as one of the Five Constants (五常: 仁 benevolence, 義 righteousness, 禮 propriety, 智 wisdom, 信 trust). Identical across 繁體 / 新字体 / 简体.

The character carries a fascinating dual meaning across CJK languages: "to trust / belief" AND "letter / message / signal". The link comes from older usage: a sealed letter or bound message was a "trusted communication" — what one person sent to another was a token of their word.

Mandarin: xìn, falling 4th tone. 相信 (xiāngxìn, to believe), 信任 (xìnrèn, to trust), 自信 (zìxìn, self-confidence), 信仰 (xìnyǎng, faith / religion), 信用 (xìnyòng, credit), 信用卡 (xìnyòngkǎ, credit card). The "letter" sense survives in: 信 (xìn, a letter), 信件 (xìnjiàn, mail / correspondence), 信封 (xìnfēng, envelope). Also: 信号 (xìnhào, signal — the technical term).

Japanese: on-reading シン (shin) is dominant — 信頼 (shinrai, trust), 信用 (shinyō, credit), 信仰 (shinkō, religious belief), 信号 (shingō, traffic light / signal — every Japanese driving lesson uses this), 通信 (tsūshin, communication / telecommunications), 自信 (jishin, self-confidence). Kun-reading まこと (makoto, sincerity) survives mostly as a male given name. The on-reading シン dominates so thoroughly that this is one of the few characters where memorizing the on-reading covers nearly all real-world use.

通信 (tsūshin) is a particularly important compound — it names the entire telecommunications industry across CJK. 通信会社 (tsūshin gaisha) means "telecom company"; the same compound 通信公司 (tōngxìn gōngsī) works in Mandarin.

Memory aid: a person and their word, side by side. The word IS the person.