Each turn has a 45-second timer. Miss two turns in a row and you're sat out (auto-fold). Press "I'm back" before the next deal, or you are removed from the table and your chips go back to your wallet. Not your turn yet? Tap Auto fold, Auto check or Auto call to choose in advance.
"Lobby" keeps your seat; "Leave table" returns your chips to your wallet. To turn chips into money, use Cash out in your wallet on the lobby page.