En este vídeo Sandra Handler te enseña el significado del idiom y su uso en contexto de una forma práctica.

Break out of one’s shell – Salir del cascarón.

Think of a turtle. A turtle has a shell. When it thinks it’s in danger, it hides in its shell and only comes out when it thinks the danger has passed. We use this expression to describe the transition when a person who is shy or quiet gains confidence and starts to participate more.

Beth has really come out of her shell this year. – Beth realmente ha salido del cascarón este año.

I hope our son comes out of his shell at camp this summer.  –  Espero que nuestro hijo salga del cascarón este verano.

You need to come out of your shell.  –  Necesitas salir del cascarón.