It would have 'melted your heart' [=it would have filled you with compassion, sympathy, etc.] to see her lying in that hospital bed.
Her anger 'melted away' when she saw that he was truly sorry.
He seemed to 'melt into' thin air. [=he seemed to disappear]
(..) God! How do you...? MARTY: What made you change your mind? Last night, Darth Vader came down from planet Vulcan... ...and told me that if I didn't take Lorraine out, he'd melt my brain. Let's just keep this brain-melting stuff to ourselves, okay? - Yeah. Yeah. - All right. There she is. (..)1985 Back to the Future