Nobody keeps a record of which cards players think of first, so this estimates it. A card counts as well known if it is played a lot in Commander (its EDHREC rank), has been reprinted often, or has ever been banned or restricted. Your score comes from where your card sits among everything that fits the prompt. The estimate is reliable about the obvious answers and rougher about the forgotten ones, and it leans towards a Commander player's idea of famous.

That is only the starting point. Every answer played here is counted (the card and the prompt, nothing about who gave it), and as answers accumulate the scores follow what people actually name instead of the estimate.

Each prompt is a real Scryfall search, put together from parts (a colour, a type, something the card does, a set, an artist) and kept only if between 100 and 1,000 cards fit. Fewer than that and there is no room for a clever answer; more and almost anything would do. After you answer, the search itself is shown so you can see everything that fit. There are 75 hands a day and every player gets the same ones in the same order.