Board Game Arena

could someone explain the roles mechanism tag? I don't know why Puerto Rico has the tag when Pandemic doesn't

Because only the developer of the game or the admins can manage the tag.
If the developer does not care, then the tag list might be completely empty as well.

I don't know if "Role mechanism" is defined anywhere... But if I had to guess, I would assume it means that a game mechanism is to select a role on your turn which then might (or might not) be executed by all players. That would be the case in Puerto Rico (and also race/roll for the galaxy, Eminent domain) but not in Pandemic.
But I guess it isn't properly defined anywhere at all - so some other games that have no role selection but some sort of roles might have also set the tag... the different people setting tags for their game have probably very different idea what the tags mean