The truth of dukkha is the basic insight that life in this mundane world, with its clinging and craving to impermanent states and things is dukkha, and unsatisfactory. Dukkha can be translated as "incapable of satisfying",Ajahn Sumedho, The First Noble Truth (nb: links to index-page; click "The First Noble Truth" for correct page. "the unsatisfactory nature and the general insecurity of all conditioned phenomena"; or "painful". Dukkha is most commonly translated as "suffering", but this is inaccurate, since it refers not to episodic suffering, but to the intrinsically unsatisfactory nature of temporary states and things, including pleasant but temporary experiences. We expect happiness from states and things which are impermanent, and therefore cannot attain real happiness.
