The Basic plan gives full access to the "Watch" section. This lets you watch videos with the assistance of the interactive captions. People usually prefer this if they have a way of reviewing vocab that they are already used to and prefer. They use FluentU as a source of new input and then review with their own system. You'll also have access to the "My Vocab" and "Ready for Review" sections, which you can use to review on FluentU itself.
The Plus plan gives full access to everything, including the "Quiz" section. It gives you exercises on the vocab you've seen in the videos. Quiz mode lets you learn vocab through video context, and through personalization (the prompts are personalized for you based your history on FluentU). And reviews are scheduled using spaced repetition so that you can focus on learning (instead of managing your learning). You also get access to personalized flashcards and printable PDFs of each video's transcript.