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]
(..) to do something. What are you doing? Get down from there! -I'm gonna teach that boy a lesson. -Yeah, sure. You go ahead. Melt him with your scary laser. Be careful with that! It's extremely dangerous. He's lighting it! He's lighting it! Hit the dirt! Look out! (..)1995 Toy Story