你好 (nǐ hǎo) is the standard greeting in Mandarin, literally 'you good/well.' 你 means 'you' and 好 means 'good' or 'well,' so the phrase is really more like 'you are well' used as a hello, rather than a question.

A few things worth knowing:

• 你好 is polite and neutral, fine for strangers, teachers, or formal settings. It's actually often seen as slightly formal or textbook-ish among close friends, who might just say 嗨 (hāi, 'hi') or use the person's name.
• To greet a group, you don't say 你好 to each person — you'd say 大家好 (dàjiā hǎo, 'hello everyone').
• For someone older or someone you respect, 您好 (nín hǎo) is more polite, using the formal 'you' 您.
• It doesn't really function as a real question about someone's wellbeing, so don't expect a literal answer about how someone feels. If you want to ask that, you'd say something like 你好吗? (nǐ hǎo ma?, 'how are you?').
• Repeating it twice, 你好你好, can sound a bit eager or overly enthusiastic, like an exaggerated greeting.

Example: 你好,我是马克。 (Nǐ hǎo, wǒ shì Mǎkè.) — 'Hello, I'm Mark.'