"What do you do?" is a phrase, sometimes used as a ritual phrase, that can be used (generally by a GM) to prompt a player to participate in the role-playing game by describing how their character reacts in response to a situation.
Asking "What do you do?" is part of the rules in many Powered by the Apocalypse games, but is often good practice in other games to both clarify when a player is expected to contribute and encourage them to do so.
"What do you do?" is usually an open question posed by the GM after describing what is happening or about to happen (e.g. "A giant raises its foot and prepares to stomp on your friend. What do you do?"), but depending on the system may be used by GMs to encourage players to pick from a finite list of options with explicit (usually negative) outcomes (e.g. "You can either either rescue your friend and be trapped, or leave them to their fate and chase the villain. What do you do?"). In the latter case, the outcomes of each choice may be fixed by the GM's description, with the action being resolved by the player's choice (a form of judgemental resolution).