Haechan's nickname. His stage name is “Haechan” or 해찬 in korean.
Hae (해) means “Sun” while Chan (찬) means “to be filled with” or “full” so that’s why he’s also called as “Full Sun”.
A: Haechan is an amazing singer! His tone of voice really is something else!
B: I know right! Full Sun is amazing!