Short answers, pulled from the story.
Avidyā commonly translates as unseeing or ignorance, referring to one's ignorance or misconceptions about the nature of metaphysical reality, especially the doctrines of impermanence and anatta. Buddhist teaching treats it as the root cause of dukkha, suffering and unsatisfactoriness.
Buddhist teaching holds that Avidya, rather than sin, is the basic root of dukkha, and that removing Avidya leads to overcoming dukkha. Peter Harvey describes it as not a lack of information but a more deep seated misperception of reality, and Gethin calls it a positive misconception.
Avidyā is a compound of the Sanskrit prefix a- and vidya, meaning not vidya, built on the root vid, meaning to see or to know. The word ultimately traces to the Proto-Indo-European root *weid-, a cognate of the Latin vidēre and the English word wit.
Theravada Abhidharma counts Avidya among the ten fetters and treats it as equivalent to moha, while Bhikkhu Bodhi compares it to a cataract obscuring perception of the true nature of things. Mahayana teaching counts Avidya as one of three poisons and six root kleshas, breakable only through insight into Emptiness, or sunyata, while Vajrayana treats it as a fetter removed through a Tantric path toward liberation in a single lifetime.
Avidyā holds the first position among the twelve nidanas, the twelve links of dependent origination presented in the Samyutta Nikaya at II.2-4 and the Digha Nikaya at II.55-63. The twelfth link, Jarāmaraṇa, old age and death, triggers the dependent origination of Avidyā again, recreating an unending cycle of dukkha.
Avidya is most discussed in relation to the Anatta, or Anatman, doctrine that only non-Self exists, and the Anicca doctrine that reality is impermanent. Buddhist teaching holds that Avidya causes misconceptions about both self and permanence.