The door to this room is locked and no spell or magical device appears to work to open it. Harry tried his magical penknife, which would open any lock, but the blade melted away and the door remained solidly locked (OP34). Inside this room, according to Dumbledore, is the most powerful magic of all: love. It is this magic, generated by his mother's sacrifice, that protects Harry. It is also this magic that drove Voldemort out of Harry when the Dark Lord tried to possess him (OP37).
Rowling's comments on the "Love Room":
"What's in the Love Room. It's the place where they study what Love means. So that room, I believe, would have at its center, a kind of fountain or well containing a love potion, a very powerful love potion. You know that the first time they ever enter Slughorn's Potions class, and he starts talking about Amortentia, the Love Potion, and he says it's the most dangerous one in the room. That's what they would've found in the Love Room. So you would see wizards and witches taking it, they would study the effects. The room, of course, has to be locked. And you know, again, there's this thread running through the books, what Love does, and it raises people to the heights of absolute heroism, as in Lily, Harry, Neville. And it also leads them into acts of foolishness and even evil, which is Bellatrix and also Dumbledore. He became foolish. He lost his center, his moral center, when he became infatuated. So that's what it does, that's what makes it dangerous." (TLC2)
Amortentia (the Love Potion)