Is that an one sentence? Anyway "Saengil Chukhahaeyo" means happy birthday. "Maknae" means a youngest child or sister or brother....... and "Seororo"? If you can show me that word in Korean then I could answer it. But by the pronunciation...I am not quite sure what it means.,.. Anyway hope I did help you :) Good luck for studying Korean!

Jenna Park is right. I guess 'Seororo' is someone's name.