This is the 809th most frequent Turkish word.
herkesin
"Everyone's" or "of everyone."
In this sentence, 'herkesin' is the possessive form of 'herkes' (everyone), indicating that the opinion belongs to everyone.
Herkesin fikri farklı olabilir.
Everyone's opinion might be different.
Here, 'herkesin' indicates that the subject (everyone) is expected to have knowledge about the song.
Bu şarkıyı herkesin bilmesi gerekir.
Everyone should know this song.
In this context, 'herkesin' demonstrates possession, as it relates to the happiness that belongs to everyone.