what is invitation letter and how i make it i am from china i want to make for my boy friend ..

Hi Sir i am Sun xue i am from china i have one pakistani boy friend he is working in Dubai i want he come here in china in my home i want he stay here with me i want make Visa for him but yesterday he told me about invitation letter so kindly plzz can you tell me what is invitation letter and how i can make it for him