Generics are templates for generating other types. These can be classes, objects, interfaces and even functions, arrays, records. It is a concept that comes from C++, where it is deeply integrated in the language. As of version 2.2, Free Pascal also officially has support for templates or Generics. They are implemented as a kind of macro which is stored in the unit files that the compiler generates, and which is replayed as soon as a generic class is specialized.
Creating and using generics is a 2-phase process.