Just Two Simple Words ...
 

Shut up.

yessum

Game management to me means:
knowing the rules, but also knowing when to enforce them.
Knowing when to talk, knowing when to listen, and knowing when to take care of business.
Having clock and situational awareness.
Anticipating different coaching strategies, so you can know what a coach or team will do before they actually do it.
Crew communication, both verbal and nonverbal.
Trust your partner to get the right calls, in the right area.
Being Trustworthy so your partner(s) can do the same.
Then knowing when to reach and get that crew saving call.
Be patient on the whistle, and see the development middle and end of a play.
Eye contact.
Crisp Mechanics.
Look the part
And most importantly:

DFIU!!

In one sentence, it is having control of all facets of the game so that the AD doesn't have to worry that the game is going to blow up.

It includes dealing with players and coaches, making the early call that sets the tone for the night and dealing with bad behavior appropriately.

I was at a game with two very good teams. I was sitting with a couple experienced refs. Team A jumped out to a large lead of 20 points. After one foul call (which was kind of a 50/50 call) in Team B's favor, the guys I was sitting with said "good game management call!" I asked them what they meant, and they told me.

I want to know, what does this sort of thing mean to you?

You can't do this too early in the game. If it was two good teams, then anytime in about the first 3 quarters is too early, even with a 20-point lead.

When appropriate, though, it means that you have observed that the definition of "advantage / disadvantage" isn't what you thought it was.

It means a team is realistically out of the game and you have a play with "marginal" contact that you might have passed on in the 1st 3 quarters but you put a whistle on it now towards the end of the game to keep the losing coach from finding a reason to get in your ear.

The 50/50 foul call I mentioned happened in the second quarter. Team B made a run in the second half and made it a three-point game by the end.

So, only call the marginal stuff if a team is clearly not going to win?

I'd be very careful about using the term "marginal stuff".

And, was there a question about who the foul was on, or whether there was an advantage gained?

Part of game management is also knowing when to "tighten up" and when to "let them play." (not that this has anything to do with the specific play)