Willy Loman, in Arthur Miller's play "The Death of a Salesman", states the following: "be liked and you will never want."
Basically, what Willy is saying here, is no different from how people are regarded today--62 years later. People who are liked get more out of those who like them. Whether it is sales (like Loman) or friendship. It is simply easier to do something, to help someone out, when you like them.
Here, Loman is simply stating the facts in life. During a conversation with his wife, Linda, Willy is simply stating that he cannot understand how he cannot make the sales he so desperately needs.
Later, in Act One, the quote in question appears:
the man who makes an appearance in the business world, the man who creates a personal interest, is the man who gets ahead. Be liked and you will never want." Act 1, Part 3
Willy understands that the man who makes himself known, the man who, in essence, "plays the part", is the man who will be liked by all others. In the end, when one is liked by others, it is simply easier to get what they need from them.