Complex sentence: The surname "Ryu" (also Yu (유), commonly romanized "Yoo") comes from the character meaning willow tree, hence the lineage is also called "willow Ryu" (버들류씨).
Split into simpler sentences: The surname "Ryu" (also Yu (유), commonly romanized "Yoo") comes from the character meaning willow tree. Hence, the lineage is also called "willow Ryu" (버들류씨).