When you're dumbfounded, you're amazed. Being dumbfounded is an extreme form of being surprised or caught off guard.
Being dumbfounded is not something that happens every day: this is a word that means something similar to astonished and gobsmacked. Only things that are extraordinary can dumbfound you. This word plays on an older meaning of dumb that means unable to speak. When you're dumbfounded, you're so astounded and shocked that you can't even speak. Save this word for occasions that are truly unusual and amazing.