In language learning, input refers to the language that a learner is exposed to through reading, listening, or observing. This contrasts to output, which is language that learners actively produce through speaking or writing. Both are vital to well-rounded language learning routine.
Comprehensible Input is language that is slightly above the learner's current level of understanding. It mimics how we acquire our first language: through exposure to language we can make sense of, even if we don't understand every word or structure.
The Videos page is sorted by Comprehensible Input Percentage, which is the percentage of words in a given video that a user knows. This measure is shown to the left of each video title and can be filitered on the sidebar. The ideal percentage comes down to personal preference and generally ranges anywhere from 80% to 98%. On the Watch page, you are given a custom queue of video clips tailored to a percentage of your choosing.
The Learn page shows the most common words in our database that the user does not already know. Traditionally, databases of commons words in a given langauge take data from a variety of different input sources like books. By narrowing this down to videos, you end up learning vocabulary that is most applicable to spoken language. By clicking a video on the Videos page, you can also see the most common words in that video that you do not already know. On the Review page, you can review words that you've already been exposed to via flashcards based on the FSRS algorithm. Reviewing vocaulary in this way is vital to long-term memory and is highly recommended.