Brainliest is a feature that simply states that the asker of the question thought this answer was the best. If your answer is marked Brainliest, then you get 1/2 extra points. (If the question is worth 5 points, and your answer is picked, then you get 3 more points.) Your answer is also moved to the top. Brainliests are also required to move up in ranks (which are just bragging rights). To mark someone Brainliest, there must first be 2 answers on your question. Then, click the crown on the bottom of their answer.
