Basically, time is a big circle, a loop. Everything is predetermined, and the three-eyed raven knows all the events in this big loop and how they are connected.
So the three eyed raven knew Bran would make the mistake of touching the wrong tree root and will end up in front of the white walkers. He also knew that once that happens, his time will be up and Bran will have to “become” him. He knew then, that the white walkers will come for Bran, he’ll be killed by the wight king and Hodor will ultimately help Bran and Meera escape by holding the door.
So the three-eyed Raven takes Bran to Winterfell in the past, where Bran can come face to face with Hodor of the past. Time is a big loop, and by taking Bran in the past, the raven connects the present and past to each other.
Bran is somehow, in the middle of the past and present.
When the appropriate time comes, the three eyed raven encourages Bran to “listen” to Meera. So Bran does. He understands he has to warg into Hodor so that Hodor can help them. Bran’s act of warging into Hodor, connects Hodor’s past and present selves. Making the loop something like this:
So when the present time Hodor, hears Meera say “hold the door”, and starts to stop the door from opening, the past time Hodor can hear her too. And not just hear, but sort of experience it. Warging definitely has a toll on the person, but this warging is extreme, as it is across two different times. The extremity of it, screws up with Hodor’s brain in the past, who is somehow experiencing his last few moments without really realizing what is happening. He keeps hearing ‘Hold the door’ and the extreme experience of holding the door against the white walkers as they hurt him, has such a psychologically damaging effect on Wylis, that he ends up saying ‘Hodor’ for the rest of his life, and the growth of his brain stops, and he basically goes kinda insane.
Aries:*thrusts his dagger in the table* Then you'll be fucking your own bride with a wooden cock.
Taurus:That's what I do. I drink and I know things.
Gemini:When I was seven, I saw a servant girl bathing in the river. I stole her robe and she was forced to return to the castle naked and in tears. I close my eyes, but I can still see her tits bouncing...
Cancer:Oh, Lancel, tell my friend Bronn to please kill you if anything should happen to me.
Leo:I am the god of tits and wine. I shall build a shrine to myself at the next brothel I visit.
Virgo:I don't pay you to put evil notions in my head.
Libra:I have a tender spot in my heart for cripples and bastards and broken things.
Scorpio:I will hurt you for this. A day will come when you think you are safe and happy, and your joy will turn to ashes in your mouth. And you will know the debt is paid.
Sagittarius:Next time I have and idea like that, punch me in the face.
Capricorn:I'm not questioning your honor, Lord Janos. I'm denying its existence.
Aquarius:Never forget what you are. The rest of the world will not. Wear it like armor, and it can never be used to hurt you.
Pisces:A mind needs books like a sword needs a whetstone.