What does the "Guard" option do in Campaign?

I don't what that option does. For those who don't know what I am talking about, it's that shield icon next to admiral portrait of the selected fleet in the main map. I have toggled it on and off and I have no idea what it has accomplished.

Pretty sure Guard mode makes you FIX BAYONETS.

I think this is just a way to mark a fleet when you don't want to move it
Each turn I check all my fleets, if they have movement points I know I have to give them order
If I want a fleet to wait to defend on invasion I enable the guard mode
This way when all my fleets have 0 action point, have the guard symbol or the battle symbol I know I can click on "end turn"

my money is on they wanted to have a notification for idle fleets like civs does for its units but then realized it would be more work.

Does it stop enemy fleets moving through the system and force then to fight?