SPOILER ALERT!!!!!
Basically, the 5 of them were childhood friends who did everything together, and were almost inseparable, always playing together and what-not. Until one day, when menma gets in an accident and is killed. This causes the group to split apart and live out their lives separately.
Then, during one summer, Menma appears to Jintan (the main protagonist), and him alone, and she believes that to get to heaven, she needs to have her list wish fulfilled, which coincidentally, she can't remember.
The ending is truly sad, after you watch the entire anime, as you begin to understand how everyone couldn't really let go of her, and that she never was truly gone. The reason for the ending being like that is because Menma begins to fade from Jintan as well, until the end, when the group truly get to say goodbye to her.
Truly a sad anime, I cried hard. 10/10 would watch again