For Kierkegaard's author, Vigilius Haufniensis, anxiety/dread/angst (depending on the translation and context) is unfocused fear. Haufniensis uses the example of a man standing on the edge of a tall building or cliff. From this height he can see all the possibilities of life. He's reflecting on what he could become if he only threw himself into the power of his own choice. As long as he stands there he stands at the crossroads of life, unable to make a decision and live within its boundaries. The mere fact that one has the possibility and freedom to do something, even the most terrifying of possibilities, triggers immense feelings of dread. Haufniensis called this our "dizziness of freedom."
Anxiety may be compared with dizziness. He whose eye happens to look down into the yawning abyss becomes dizzy. But what is the reason for this? It is just as much in his own eye as in the abyss, for suppose he had not looked down. Hence, anxiety is the dizziness of freedom, which emerges when the spirit wants to posit the synthesis and freedom looks down into its own possibility, laying hold of finiteness to support itself. Freedom succumbs to dizziness. Further than this, psychology cannot and will not go. In that very moment everything is changed, and freedom, when it again rises, sees that it is guilty. Between these two moments lies the leap, which no science has explained and which no science can explain. He who becomes guilty in anxiety becomes as ambiguously guilty as it is possible to become. Vigilius Haufniensis, The Concept of Anxiety p. 61
In The Concept of Anxiety, Haufniensis focuses on the first anxiety experienced by man: Adam's choice to eat from God's forbidden tree of knowledge or not. Since the concepts of good and evil did not come into existence before Adam ate the fruit, which is now dubbed original sin, Adam had no concept of good and evil, and did not know that eating from the tree was evil. What he did know was that God told him not to eat from the tree. The anxiety comes from the fact that God's prohibition itself implies that Adam is free and that he could choose to obey God or not. After Adam ate from the tree, sin was born. So, according to Kierkegaard, anxiety precedes sin, and it is anxiety that leads Adam to sin. Haufniensis mentions that anxiety is the presupposition for hereditary sin.
However, Haufniensis mentions that anxiety is a way for humanity to be saved as well. Anxiety informs us of our choices, our self-awareness and personal responsibility, and brings us from a state of un-self-conscious immediacy to self-conscious reflection. (Jean-Paul Sartre calls these terms pre-reflexive consciousness and reflexive consciousness.) An individual becomes truly aware of their potential through the experience of dread. So, anxiety may be a possibility for sin, but anxiety can also be a recognition or realization of one's true identity and freedoms.
Whoever has learned to be anxious in the right way has learned the ultimate. … Anxiety is freedom’s possibility, and only such anxiety is through faith absolutely educative, because it consumes all finite ends and discovers all their deceptiveness. And no Grand Inquisitor has such dreadful torments in readiness as anxiety has, and no secret agent knows as cunningly as anxiety to attack his suspect in his weakest moment or to make alluring the trap in which he will be caught, and no discerning judge understands how to interrogate and examine the accused as does anxiety, which never lets the accused escape, neither through amusement, nor by noise, nor during work, neither by day nor by night. — Vigilius Haufniensis, The Concept of Anxiety p. 155-156