Sometimes people shorten it to Ano Hana. But the title literally means "We Still Don't Know the Name of the Flower We Saw That Day".
Annnd I'll quote this from wikipedia because it's a good plot summary. XD
Quote:
A group of six childhood friends drift apart after one of them, Meiko "Menma" Honma, (the girl with the white hair) dies in an accident. Several years after the incident, the leader of the group, Jinta Yadomi, has withdrawn from society and lives as a recluse. One summer day, an older looking Menma appears before him and asks to have a wish granted, though she does not remember what it is. Forcing him to gather his estranged friends to help Menma pass on. But they face trouble of getting the group believe him, Menma's parents and the conflicts hidden within the group.
Posted: Sat Oct 01, 2011 2:17 pm
I've seen that anime and it was very sad. :< However, it's definitely a recommended anime to watch regardless of it being sad.