What's the difference between a "flight line" and a "tarmac"?
The nuance is between aircraft that have been / will be flying imminently, and aircraft that are more than a few hours away from doing so.


Apron (U.S., ramp) is where aircraft refuel, load/unload, etc. 
Flight line is, in the popular press, where aircraft are parked, maintained, and displayed.  This usage is duplicated by aeromodelling clubs, although their time scale is closer to minutes than hours. (Wikipedia claims, without substantiation, that flight line is the U.S. military term for tarmac, ahem, apron.)
Tarmac (U.S., asphalt) is a paving material, one possible surface for runways and roads.  It's not an aviation term.  Informally, it means apron: we hear about tarmac delays, never about flight line delays.