it is not really a condition as such, it is not something certain people 'have' - it is something that can happen to anyone at any time.
it is simply the bodies way of preventing you from acting out what is happening in your dreams, physically, for real.
this is necessary obviously as we would all be in danger, and it is the lack of this that causes some people to sleepwalk or sleep eat etc.
it happens when your dream slips into a lucid state of heightened reality - where everything seems extra real and you feel things that you would not normally notice in ordinary dreams, such as wind in your hair, an itch, soft carpet under your feet - you feel all this as though it is real.
in this state you are usually dreaming of yourself as you actually are - in bed asleep - and thats why you think you are awake, and therefore try to move - the terror at not being able to move is what creates the feelings of fear, pressure etc.
i once was terrified at my "predator" style hand. you know the sort of invisible shape the monster has? well my hand was like that and i was looking at it on top of the duvet, unable to move it, freaking out, eventually i moved my hand a tiny bit, woke up and found my real hand was under the duvet, not on top.