The word "만세(萬歲) literally refers to "for 10,000 years".
It means that "Long live something/somebody! " (e.g. country, nation, or the King/Queen)" In English,
For examples,
대한 독립(大韓 獨立), 만세(萬歲)!
Long live the Independence of Korea!
대왕 폐하, 만세!
Long live the King!