Hermia's dream occurs at the end of Act II scene 2, and is rather ironic, because she narrates the dream that she has once Lysander has been bewitched to fall in love with Helena and has abandoned his former lover to pursue Helena through the woods. When we read the content of Hermia's dream, it assumes a grim significance, because we can see how what she dreamed has actually happened, at least metaphorically:
Methought a serpent ate my heart away,
And you sat smiling at his cruel prey.
The way in which the narration of this dream corresponds with her realisation that Lysander has actually left her makes it a rather cruel awakening indeed. The dream that Hermia has experienced has become true thanks to the way in which Puck has mistakenly enchanted Lysander and the unexpected arrival of Helena.