'Prego' is a general term of politeness and openness that can have various equivalents in English, depending on the context. It is used as a response to 'Grazie', but it isn't only 'You're welcome'.
It can correspond to 'Of course', 'Go ahead', 'Please do', 'Be my guest', 'There you go', 'Help yourself', and numerous other untranslatable phrases. Context is everything!