“Fresh air” means the air outside. The air inside of a building can become “stale” and unpleasant, so it’s nice to go outside where the air is often cool and refreshing. You probably wouldn’t use this expression if the outside climate is very hot and humid, because that isn’t very refreshing.

We need some fresh air in here.

I think some fresh air will make me feel better.

To "get some fresh air" means to open a window or go outside, so that you can breathe air that comes from outdoors.

Turn off the computer, go outside, get some fresh air, and relax.