Dukkha is associated with the First Noble Truth of Buddhism. That truth holds that life itself is dukkha. The most usual translation of this word is "suffering."
However, this word is not really a perfect translation. Buddhists do not believe that all of life is physical pain or torment. Instead, they believe that all of life is uncomfortable for us because we constantly want things that we cannot have. Therefore, we might translate dukkha as "discomfort" or "frustration."
We can even translate dukkha as "impermanent" or "temporary." This is because one aspect of the dukkha of life is that fact that nothing that is pleasant remains that way. If we love, for example the way our children are today, that is dukkha because they will soon change and become something different. In that way, dukkha can simply mean "impermanent."
So, there are many ways to translate this word. The basic idea behind it is that we are always unhappy and dissatisfied in life because we always either want things we cannot have or want things to stay as they are even though they will always change.