Pressing the heart icon next to a quiz will Recommend it. This does several things - firstly, it stores the quiz in your Library so that you can access it again later. Secondly, it will notify the author to let them know that you liked their quiz - this is a nice bit of positive feedback that will hopefully encourage them to write more quizzes like that! Finally, it also lets our algorithms know that this is a quiz relevant to you, which we take into account when recommending new quizzes for you to practice.
You can un-recommend a quiz at any time and the author will not be notified.