Spaced Repetition Software (SRS) helps you remember things easily by letting you review something just as you're about to forget it. 

FluentU has an SRS feature known as "Ready for Review." 

On your homepage, you will periodically see a list of words that are "Ready for Review." 

FluentU is using its SRS algorithm to show you words at the perfect time for you to review them. 

FluentU's SRS algorithm is based on a variation of the Supermemo SM2 algorithm (the same one that Anki is based on).