What is a sub assembly?

Yes, that's a subassembly. For a tank kit, the turret will be one subassembly and the hull another. Tracks could be another if they are of the multipiece style and not one piece vinyl.

Even the main gun on a turret is a subassembly of the turret, which is a subassembly of the tank kit.

Cars, the tires could be a subassembly, as well as the interior tub, engine, etc.

It's when a bunch of union guys get together and build a submarine. 

Ha, was thinking the same kind of joke.  
putting a uboat together 

OP, you hit the nail on the head.  

Hello!

We can connoct more than one definition here - but it's mostly up to you, how you manage your build. The basic idea is to take one or more parts of the model and work on them separately  for some time - a small model to be installed in a bigger model - so to say. The level of finish of that small model is also quite flexible - it might be painted and decaled before installation, or it might be just the assembly that's being done. But I'd say you alredy get the idea - so good luck with your subassemblies and have a nice day!

Paweł

I usually assemble an airplane, except for landing gear in a retract plane, before painting.  I find it easier to mask wheel wells than to mask an installed LG. 

Cockpits and cowled engines have to be painted before installation, of course.  To mask off radial aircraft engines, I cut a thin cardboard or paper disk about a sixteenth larger than cowl opening, then cut a radial line through the disk.  That allows me to bend the piece slightly into a shallow cone to insert it into the cowl.