It means not to be decided¹ yet.

eg: The company is going to build a new factory, but the location is still up in the air.
   
    They are eager to see the band U2 in concert, but their tour schedule is very much up in the air.

Note:

Be up in the air 的意思为悬而未决的，还未定下来的。

例：公司将要建一座新工厂，但是地址还未选定。

    他们希望能在演唱会中看到U2，但是他们的演出行程非常不确定。