There are multiple flashbacks in this episode.
We learn how Rei met and became friends with Akari, Hina, and Momo.
In a similar theme to Your Lie in April, Rei’s world changes from gray-scale to color. There’s also the theme of mourning with the Obon festival, both Rei and the sisters are mourning people close to them.
From the Deck of the Matcha Latte 