When something stupefies you, it makes you feel very surprised or shocked, like you can't believe what just happened.
If something stupefies you, it is usually something very unexpected or unbelievable.
Examples:
The magician's trick stupefies the audience.
The loud noise stupefies the cat.
The news stupefies everyone.
The size of the cake stupefies me.
The number of stars in the sky stupefies me.
The idea of flying without wings stupefies me.