What does "It is a means to an end, not an end in itself" mean?

An "end" or "end in itself" is the end result, the ultimate goal, the final conclusion. A "means to an end", therefore, is a way of getting to a given goal.

So for example, if I want to lose ten pounds, I might start running to lose weight. For me, running is a means (the very act of running) to an end (losing the weight). I could also start dieting; I don't want to eat less but it is a means to an end. If, however, I actually like running, the act of running is itself the end, so the expression would not be appropriate.

A related expression is "The ends justify the means." This phrase is used when the end result justifies whatever action was used to get there. The main character in a movie who wants to avenge a murdered loved one might go on a killing rampage to get at the murderer, and you might say that for her, the ends justified the means.

The idiom "A means to an end" differentiates between an end goal and the means or methods and actions used to reach that goal.

If I wanted to get a job, a means to that end might be writing my resume. When I found a job my end would be achieved, the goal I had in mind. But the end was never to write myself a good resume, that was only a means to the end, getting a job.