[consolegaming]

I'm not sure what the technical term for it is but it's used for PHP's OOP (object-oriented programming). $vbulletin is a class and to access that classes members (variables of the class) or functions you can use the -> operator. So that just means that GPC['moderatorid'] is a member of the $vbulletin class. Quite a few programming languages use the same operator for this purpose.

EDIT: Posted this before seeing Happy's response. I'm not really sure that's a valid response. I don't know much about vBulletin itself/their functions etc but as far as I can tell the GPC there isn't a function it's just an array. I'm presuming this clean function that you mention sets this array up though. Plus I certainly weren't aware of -> being a custom vB command lol. It's part of PHP's programming language.