This is the 540th most frequent Russian word.
самое
"Самое" means "the most" or "the very" in English.
Here, "самое" is used as part of the superlative form of the adjective "красивое," meaning "the most beautiful."
Это самое красивое место.
This is the most beautiful place.
In this sentence, "самое" emphasizes the superlative degree of "интересное," signifying "the most interesting."
Скажи мне самое интересное.
Tell me the most interesting thing.
"Самое" here is used to provide emphasis, denoting that it is specifically 'that very' dress being referred to.