Called tiles become public.
Your melds (Eyes, Chow, Pung, Kong) can either be made directly in your hand (concealed) or by revealing your meld's tiles when you call another player's discarded tile. Tiles in your hand all start as concealed: only you can see them. When you call another player's discard, the set you make becomes open and visible to everyone.
Building your winning hand with concealed tiles prevents other players from knowing your tiles. Building public "open" melds lets you utilize other player's discarded tiles but reveals information. You'll learn more about this in later sections.