If you play the game the movie is a great!! It's so nice being able to see the characters become alive on screen. The movie follows an original storyline but features all 20 game original characters so background information is required to get full enjoyment. If you don't want to play the game I recommend watching the "journey to bloom" series on youtube posted by the official project sekai account. It's 5 short videos that introduces the 5 bands (it summarizes the introductions in the game). For extra understanding I recommend also readingthe wiki on each group and each character to understand the relationships shown in the movie.