Learn Italian: Siete stati bravissimi, il concerto è stato un capolavoro! - You were really good, the concert was a masterpiece!

To make the absolute superlative (which means "extremely" or "really"), you drop the final vowel of the adjective bravo and add the suffix -issimo. Because the verb siete stati shows we are talking to a plural "you", the ending must agree in the plural, making it bravissimi.

Why does the sentence use siete stati instead of avete stati for "you were"?

The verb essere (to be) is a bit special: it always uses itself as the helper verb in the past tense (passato prossimo). So "you were" is literally "you are been" (siete stati). Whenever essere is the helper, the past participle (stati) must match the subject in gender and number.

Could I say buonissimi instead of bravissimi to mean they were really good?

Not in this context. Bravo is used to describe people who are skilled, talented, or good at a specific activity (like performing a concert). Buono is used to describe tasty food, high-quality items, or someone who is morally kind.

Where does the word capolavoro come from?

It is a compound word made from capo (head, chief, or main) and lavoro (work). Literally meaning "chief work," it is the Italian word for "masterpiece."