I’ve worked with SuperCollider quite a bit, but not ofxPDSP, so I’d be guessing again. But, .in_mod() is a function (or a message) that is being applied to the object amp. And, if you type in_mod into the search box of the documentation (which is one of several reasons why its so amazing!), you’ll get this page that describes pdsp::Amp and its class functions, including the function pdsp::Amp::in_mod(): ofxPDSP: pdsp::Amp Class Reference.
It looks to me that ofxPDSP might have a class structure with a lot of inheritance, and that pdsp::Amp inherits from pdsp::Unit. So presumably a pdsp::Amp object can also use the class functions of pdsp::Unit. Alternatively, a class function in pdsp::Unit can be redefined by pdsp::Amp to better suit how it would specifically apply to an object of pdsp::Amp.