This classical structure is used to express an analogy. In other words, we're talking about the relationship to different things. "A is to B what C is to D" means that A and B have the same relationship that C and D have. "Air is to us" is not a complete sentence. You could make it into a full sentence like this: "Air is, to us, what we breathe" or "Air is, to us, vital for existence." ---- Three examples: 1. Anna and Barry are a married couple. Claire and Daniel are another married couple. Anna is Barry's wife. Claire is Daniel's wife. Anna is to Barry what Claire is to Daniel. And *what* are both Anna and Claire? They are their partner's wife. You could ask, "What is Claire to Daniel?" And the answer would be: "She is his wife." 2. "Paris is to France what Beijing is to China." Paris is the capital city of France. Beijing is the capital city of China. The relationship is capital city to country. 3. Sometimes this will be posed as a question. "Paris is to France as Beijing is to what?" In this case, you need to establish the relationship between the first pair to answer the question. Paris is the the capital of France, so you need to answer what country Beijing is the capital of. The answer, of course, is "China". ---- So, the implication of your example "Air is to us what water is to fish" is that we ("us") have the same relationship to air as fish have to water. What is that relationship? You might say: - Air and water are what we breathe. - Air and water are the elements we inhabit. Does that help to make sense of it?

"What" is being used as a relative pronoun. In this case, it can be translated as "比拟". In the sentence it is used to show an analogy (or corresponding relationship) between air-us and fish-water. (人类需要空气比拟为鱼需要水) A longer (and more awkward) version if the sentence is: "Air is for humans the same thing as what water is for fish." I hope this helps.

It's a complete sentence but it just wouldn't make sense if you just said "Air is to us".