The teaching the early texts present as the Buddha's own is compact and can be stated without metaphysics, though it points at something hard.
It begins with the four noble truths, cast as a physician's diagnosis: that ordinary existence is pervaded by dukkha, unsatisfactoriness, unease, suffering; that its cause is tanha, craving or thirst; that there is a cessation; and that a path leads to it. The path is the eightfold one, grouped under conduct, meditation and wisdom, and it is the practical content of the middle way between indulgence and mortification.
Under this sit two ideas that give Buddhism its distinctive edge. The first is impermanence (anicca): nothing that arises is stable, and grasping at what must change is where suffering enters. The second, more radical, is anatta, non-self, the analysis that there is no permanent, unchanging self to be found among the shifting physical and mental processes that make up a person, a direct denial of the atman that the surrounding Indian traditions affirmed. Rebirth and karma are kept from that shared world but reframed: what continues is a causal process, not a soul.
Holding these together is dependent origination (paticca-samuppada), the principle that things arise in dependence on conditions and cease when those conditions cease. How literally to read the twelve-linked chain in which it is usually expressed has been debated within Buddhism for two millennia. What is not in doubt is that the Buddha presented liberation, nirvana, the blowing out of the fires of craving, as something to be realised, not merely believed.