Complex sentence: He saw a Rồng vàng (yellow dragon) fly around on the clear blue sky, so he changed the name of Đại La to Thăng Long, meaning "Vietnam's bright and developed future".
Split into simpler sentences: He saw a Rồng vàng (yellow dragon) fly around on the clear blue sky. That's why he renamed Đại La to Thăng Long, meaning "Vietnam's bright and developed future".