Significa que no sabe qué hacer o cómo sentirse en relación a algo que pasó o supo. Bueno, si es que no es jerga.

Mario no sabe cómo reaccionar - Mario does not know how to react So, just like Sad has just explained to you, it means that Mario does not know what to do or how should he feel in that situation. I hope this helps. If you need any questions more, please, feel free to ask me. If you need Spanish classes, you can book them right here: http://www.italki.com/teachers/professional Good night

creo eso lo que senifica ( Mario does not know how to react ?)