Frank and Alice Longbottom were Aurors (Dark wizard fighters) who were tortured to insanity at the end of the last war against Voldemort. They have left behind one son, Neville, who is being raised by his extremely strict grandmother, Mrs. Longbottom. But Frank and Alice are still alive: they live in the permanent Spell Damage ward of St. Mungo's. Ron, Harry, Hermione, and Ginny stumble on Neville visiting his parents over Christmas. They see firsthand what has happened to the Longbottoms.
Harry recalls the Order of the Phoenix photograph, in which Alice Longbottom is a plump, happy-looking witch. Now, her face is thin, "her eyes seemed overlarge and her hair, which had turned white, was wispy and dead-looking. She did not seem to want to speak, or perhaps she was not able to, but she made timid motions towards Neville, holding something in her outstretched hand" (23.205). The thing Alice is holding is a bubblegum wrapper. Neville takes it from her.
This exchange between Neville and his mother is unbelievably sad. It is clear that she doesn't truly know where she is or what she is doing, but she knows that she wants to give Neville something, that he means something to her. In interviews, Rowling has stated that this is one of the few elements of the Harry Potter series based on a true story:
That idea was one of the very few that was inspired by a real event. I was told what, to me, was a very sad story by someone I know about their elderly mother who had Alzheimer's, and the elderly mother was in a closed ward. She was very severely demented and no longer recognized her son, but he went faithfully to visit her twice a week, and he used to take her sweets. That was their point of connection; she had a sweet tooth, she recognized him as a sweet-giver. That was very poignant to me. So I embroidered the story. Neville gives his mother what she wants, and (it makes me sad to think of it) she wants to give something back to him, but what she gives back to him is essentially worthless. But he still takes it as worth something because she is trying to give, so it does mean something, in emotional terms. (source)
We find that it makes it even more painful knowing that this tragic exchange between Alice and Neville is based on real events. It's among the worst things we can imagine: a mother who is too ill to recognize her own child. This scene also emphasizes to Harry that the Potter family was not the only one Voldemort tore apart. It shakes him out of his Book 5 teenage self-centeredness a bit.