Now Playing: The First Noble Truth
What is the central idea of the Buddha's First Noble Truth?

I prefer to call it (dis)stress, although that doesn't convey the full range of uses of the Pali term used, "dukkha." But it is a reasonable enough translation of dukkha. Another, more literal, translation might be "friction," and a more accurate term to use might be "painful tension." Etymologically, it might also mean "disquiet" or "unsteady," and this is certainly related to the connotation of a bumpy ride, and the change inherently in life. 

The most literal translation of dukkha would be "bad hole," which describes a misaligned wheel axle, but that phrase is not particularly helpful (unless one keeps in mind the other Buddhist metaphor for this teaching - the turning of the wheel of truth, set in motion, which connotes movement towards a better goal).

Basically, dukkha refers to all conscious states that have an edge, that feel rough, sharp, contracted, constricted, and limited [to a self]. Dukkha -- biologically, behaviorally, phenomenalogically -- conditions us, and without it, we are released into a bliss and feel free. This is not freedom as typically imagined, the common fantasy of having certain contigent conditions in the world met or overcoming outer obstacles as such, but is a direct sense of freedom that in ordinary life-circumstances is inconstant and fleeting.

The actual discourse in which the Buddha explained it didn't state that "life is pain." That's how just it's been presented to non-Buddhists, mostly by Western scholars. Instead, he lists all aspects of life that are painful. The closest he came to saying that life itself is painful is that all (normal) association between sensory, cognitive faculties and the outer world is dukkha. But he makes it clear that clinging is the cause of dukkha, not life itself. The noble truths are thus an analysis of dukkha and its release.

The full sutra is here: http://www.accesstoinsight.org/tipitaka/sn/sn56/sn56.011.than.html.

It's also worth noting that Hans Selye, the scientist who coined the term "stress" in English to describe a biological phenomenon (a concept taken from physics), said that it was a poor translation of his idea; a better word, for its negative effects on organisms, would be strain. More about him here: http://en.wikipedia.org/wiki/Hans_Selye.