A popular title for a collection; you have to check the table of contents to be sure what you're getting. Me, I got "The Light Princess," "The Giant's Heart," and "The Golden Key".
The first is a light-hearted spoof-like fairy tale, where the heroine is cursed at her christening by the uninvited guests -- to lose all gravity. Both in the sense that she has no weight and that she takes nothing seriously. There's some fun and games about that before the problem gets truly serious.
The second combines a number of fairy tale motifs with a few fillips of his own, where two children are lost in the woods and happen on the title giant.
The third is a rather mythic tale of a boy who finds a golden key at the end of the rainbow, and a girl frightened from her home by fairies, and found in the forest by a wise old grandmother, and how they set out.