When we literally think about something up in the air, we have the idea that something’s floating or flying in the sky, perhaps an airplane or a balloon. But really if someone tells you that things are up in the air it means that these things are uncertain or unsure; definite plans have not been made yet.
“Jen have you set a date for the wedding yet?”
“Not exactly, things are still up in the air and we’re not sure if our families can make it on the day we wanted. Hopefully we’ll know soon and we’ll let you know as soon as possible.”