信 (xìn) as a verb means 'to trust, to believe' — it's a firm belief in someone's honesty or in a statement's truth. It is stronger and more personal than 认为 (rènwéi 'to think, to consider'), which is a reasoned opinion, and different from 想 (xiǎng 'to think, to believe'), which is more casual or speculative. As a noun, it means 'letter, mail', distinct from 邮件 (yóujiàn 'email, mail') which covers electronic mail.