The sutra's narrative core is a monk. A king hears a Buddha teach, renounces his throne, takes the name Dharmakara, and declares what kind of Buddha he intends to become: not in general terms but as a list of forty-eight vows, each of the form "if, when I attain buddhahood, such-and-such is not so, may I not attain perfect enlightenment". He then practises for an immeasurable time and attains buddhahood as Amitabha, which means the vows are fulfilled. That is the sutra's logic, and everything Pure Land Buddhism does rests on it.
The eighteenth vow is the one the tradition built on: that beings who hear his name, direct their thought to his land in sincere faith and call on him even ten times will be born there. Read together with the rest of the sutra it makes rebirth in Sukhavati available to people with no capacity for monastic discipline or sustained meditation. The soteriology moves from self-power to other-power, from what the practitioner achieves to what the Buddha has already accomplished on his behalf, and that shift is what made the Pure Land the largest devotional movement in East Asian Buddhism.
The sutra was probably composed in the Gandhara region under the Kushans, in the first or second century CE, and it was translated into Chinese many times over six centuries. The variation between those translations is considerable, the number of vows differs between recensions, and the point is worth holding onto: the text is a tradition of transmission rather than a fixed object.