The five precepts of Buddhism are no killing, stealing, sexual misconduct, lying, or irresponsible use of alcohol.
These moral guidelines are for non-monastics.
I think most of us tend to think of the five precepts as being about the material world. After all, killing, stealing, sexual misconduct, and getting drunk are all rooted in actions of the material body. Even lying issues forth from the mouth of the body.
But what if we look at the precepts differently? What if we view them as fundamentally signals that issue forth from the mind?
If we look at them that way, then lying, which is often glossed as the least important of the five precepts, becomes the most important.
The reason is that lies send bad signals forth from the mind. And surely killing, stealing, misconduct, and getting drunk are the baddest of bad signals. Each one is a form of lying.
In a post I put up just yesterday, Ethics, morality, I outlined a simple way to understand morality as that which reduces error and increases efficiency of mental signals, both internal and external (those exchanged with others).
In Buddhism, the great barrier to enlightenment is a confused, deluded mind. Anything that generates delusion or confusion, which lying surely does, is counterproductive. While anything that reduces delusion is good.
Buddhism, of course, recognizes the need for occasional lies—such as sanitizing the truth for children—but we really do not need to lie very often. We do not always have to say everything we think or tell anyone anything they want to know; we can easily and truthfully sidestep issues like that by simply saying we would rather not say.
In a very important way, clear signaling—honest signaling—is the foundation of all morality.