The RefPtr smartpointer
Glib::RefPtr is a smartpointer. Specifically, it is a reference-counting smartpointer. You might be familiar with std::unique_ptr<> and std::shared_ptr<>, which are also smartpointers. In gtkmm-4.0 Glib::RefPtr<> is an alias for std::shared_ptr<>, which is reference-counting. Glib::RefPtr<> was introduced long before there was a reference-counting smartpointer in the C++ Standard Library.
A smartpointer acts much like a normal pointer. Here are a few examples.