Function argument contains complete function

In My C++ library code I saw following line

 m_libs[ library_name ] = lib_ptr_t( handle, [](void*d){ dlclose(d); });

lib_prt_t declare in .hpp as

using lib_ptr_t = std::unique_ptr<void,std::function<void(void*)>>;

Can anyone explain , isthere in second argument is a complete function or some thing else. I can't understand what does it mean?