As you solve LeetCode questions, you can mark them as hard, medium, or easy. The tool will then recommend questions you should review based on (1) how hard the question was for you and (2) how much time has passed since you last reviewed it. I'd recommend normally attempting LeetCode problems and just marking them as hard, medium, or easy for you at first so the tool knows which problems to recommend you review!
Here's the theory behind spaced repetition and learning if interested: https://www.codecademy.com/article/spaced-repetition