In Karen, you attempt to fool others into thinking that your review is an actual one-star review.
In more detail, each round one player takes the role of host and reads one of three types of review cards: What's Being Reviewed, Fill in the Blanks, or Write the Review. The host reads out the review or the partial review or the product to be reviewed, then they write the product, the missing words, or the review on a sheet of paper, while everyone else makes up a product, missing words, or review that sounds plausible.
The host then reads out the all the written answers, and whoever votes for the real answer scores a point, while whoever's fake answer receives a vote also scores a point. For every five points, you lose one of your stars, and whoever is first reduced from five stars to one star wins.