To make it simple you can just play along and grasp it on the fly and ask the group
for the basic instructions.
But to clarify, in Guess the Impostor, everyone is given a card with a word that you
must take turn describing without saying the exact from the card. However, not
everyon receives a card with the word. The impostor receives only a hint of what the
word is. The goal of the game is know these impostors and vote them out after a
round of describing the word. Else, the impostor wins. If you're the impostor and
you confidently guessed the word, you can win by raising your hand and saying it. If
it's wrong however, you lose.