What it means
Yes, or "it is." As a verb it links two nouns: A 是 B means A is B.
When to use it
Chinese has no word for a plain "yes" in answer to a negative question — you repeat the verb instead. 你不是学生吗？ → 是 (I am) or 不是 (I'm not).
Examples
nǐ你shì是xué学sheng生ma吗？——shì是。
Are you a student? — Yes.
zhè这shì是wǒ我de的péng朋you友。
This is my friend.
tā他shì是zhōng中guó国rén人。
He is Chinese.
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.