空气就是我们每天呼吸的东西，我们看不见它，但是没有它就不能活。

"空气" is what we breathe every day; we can't see it, but we can't live without it.

这个词也可以说一个地方的气氛，比如“空气很紧张”。

This word can also describe the atmosphere of a place, like "空气很紧张" (the air is tense).