It means sending your army to fight against a very distant enemy... You can sleep (or do whatever weird rituals are appropriate for your species) in the intermittent time, without fear of your army getting destroyed by an attack (because it isn't home). An army save is typically done against a gold mine (no chance of it mounting a proper defense), or as a field battle against a friendly player who knows you're coming and won't attack you. There are exceptions, of course...