The world is always in a 'state', you perform actions on a 'state' for example opening a door and that changes the world to a new state - the world with an open door. To get from one state to a more different state for example, the state of a world without a cup of tea to the state of the world with a cup of tea requires many transitions - boil kettle, put tea bag in cup etc etc.
Imagine there was a finite number of world states, from each state you have a number of actions you can perform. A state where the door is closed you can choose to perform the action of opening a door. Similarly in a state where the door is open you can choose 'close door'. Some actions might be quite common and available in many of the world states - opening/closing a door might be quite common but other actions might be very rare.
In the example given, imagine there are 100 possible world states and being able to shoot that force-field armoured enemy with that laser rifle is only possible in 1 out of those 100 states then that is quite rare. Shooting some other enemy might be 20 out of every 100 states.
Your world might be in a state and you have the following actions:
Shoot generic enemy (20/100)
Shoot force-field armored enemy with laser rifle (1/100)
The suggestion is that you should choose the rarest action, to take advantage of it while it is available. That's what they mean by 'Most contrained' - the action that has the pre conditions/is the rarest. It's certainly not the only choice and I see it is only one of the heuristics they suggest.
It might be worth reading a bit more about world states.