The most blatantly obvious example of symbolism in the movie is Buttercup's dream. Often, when analyzing a dream, you've got to put some work into it. Like…ooh, maybe the dark alleyway represents fear, and the twinkling lights represent hope, something like that. But Buttercup makes it really easy on us.

She just flat out dreams that Humperdinck announces their engagement (which really happens), and she walks among the people of the kingdom (which really happens). The only difference between the dream and reality is that some old hag (she was probably a perfectly lovely person in real life) screams at her and accuses her of all sorts of awful things. We think she'd probably be beheaded for such behavior in most kingdoms, but this is just a dream, so we'll overlook it.

But yeah, even here, it doesn't take much to interpret what the hag's words might mean. She's basically just speaking Buttercup's innermost fears and hang-ups out loud. Buttercup worries she's betraying Westley by agreeing to marry Humperdinck, and this random calls her out on it. When she wakes up, she's attacked by the guilt monster.

Which, considering the tone of the rest of this movie, she's probably lucky isn't an actual monster. It totally could be.