A classical poetic epithet for cherry blossoms, often used in waka and haiku. Not used in everyday conversation; modern Japanese normally uses 桜 (さくら).
See also: 桜
'Yumemigusa' is a word often seen in waka poems about cherry blossoms.
From 夢見 (yumemi, 'dreaming') + 草 (kusa, 'grass/plant'). The exact origin is uncertain, but it is a classical poetic name for cherry blossoms, possibly from the idea that the blossoms are so beautiful they seem like a dream.