Short answers, pulled from the story.
Impermanence, called Anicca in Pali and anitya in Sanskrit, is the Buddhist doctrine that all conditioned existence is transient, evanescent, and inconstant. It is one of the three marks of existence, alongside dukkha and anatta.
The three marks of existence, or trilakshana, are anicca (impermanence), dukkha (suffering), and anatta (non-self). Pali texts state that all conditioned things are impermanent and painful, and that all dhammas are without self.
Impermanence is a source of dukkha because desire for or attachment to anything that is not permanent leads to suffering. The Buddha taught that suffering ends only when craving for unstable, changing things is abandoned, relinquished, and let go.
Anicca is intimately tied to anatta, the teaching that nothing, including a person, has a permanent self or unchanging soul. Grasping both anicca and anatta together marks a step in a Buddhist's spiritual progress toward enlightenment.
Applying anicca in daily life means meeting change, such as a parent's illness or the end of a relationship, with presence rather than resistance. Suffering over change usually comes from clinging to what is pleasant rather than from the change itself.
Impermanence, or anicca, is contrasted with Nibbana, described as nicca, the reality that knows no change, decay, or death.