This is Willy's philosophy for success that has developed through his career as a salesman, and the one he passes along to his sons, unfortunately. He believes (and with some reason) that a man has to sell himself first--to get in the door--before he can sell his product; he has to make a good appearance and be personally popular to compete. The most extreme example of Willy's philosophy in action is found in his relationship with The Woman in Boston. Willy charmed, romanced, and bribed her so that she wouldn't leave him waiting with the other salesmen in the outer office and would instead show him in immediately to the boss's office. There may be some truth in Willy's philosophy as it related to his occupation, but it proved to be shallow and limited.
Willy's faith in appearances does not serve his sons well. He fails to recognize the importance of integrity, education, and hard work in achieving success, and he does not pass these values along to Biff and Happy. At the end of his life, Willy does not understand why Biff's life has turned out to be such a failure, whereas Bernard has achieved enormous success. This is perplexing to Willy because Bernard had never been "popular" in school.