Wu-wei is one of those concepts incredibly difficult to translate into English, both because of the allusive nature of Chinese and because the philosophical concepts it embodies are so foreign to most post-Socratic Western thought. Non-action is the (relatively) literal translation, but that's caused a great deal of misconception about Taoist thought over the years. getzburg's translation of "avoidance of meddling in the natural course of things" is closer to the mark, but that still seems a bit off.

The best translation I can give of wu-wei is "minimal action" or possibly "action only within the Tao", but even that requires some backstory and explication. The basic ideal of the Taoist sage, as expressed in the Tao Te Ching was somebody who acted only when the time was exactly right, and did it in a way which was totally minimal, yet totally decisive, because his actions followed the course of the Tao. Nobody would even realize that he had acted, because his actions were so much in accord with nature. When the Taoist king acted, the people would say, "We did it ourselves."

If this still seems a bit foggy, consider the story Chuang Tzu used to illustrate the point. He described a butcher, who was so practiced in his art, and knew the anatomy of his stock so well, that whenever he made a cut, the meat immediately cleaved all the way through with one stroke. The butcher never needed to sharpen his cleaver, because it always cleaved through with the first stroke, and never encountered anything to dull it. This is wu-wei in a nutshell, action which is minimal, and seemingly requires the least effort, but at the same time it is totally effective and encounters no resistance.