The word dukkha is significant in Buddhism because of its association with the First Noble Truth -- that life is dukkha. To understand what the Buddha meant, it's important to understand what dukkha means.
The word usually is translated into English as "suffering." But it also means temporary, limited and imperfect. In the Buddhist sense, it refers to anything that is conditioned. Something that is conditioned is not absolute or independent of other things.
Thus, something beautiful and pleasant is dukkha, because it will end. For example, a new sports car is dukkha, because eventually it will be a rustbucket.
Anything formed of the Five Skandhas is dukkha. When the Buddha said that "life is dukkha," he didn't mean that life contains dukkha. He meant exactly that life is dukkha. Life is conditioned. Life is temporary and imperfect.
Please see "Life Is Suffering? What Does That Mean?" for more discussion of dukkha.
Alternate Spellings: Duhkha