Shukuzen is one of the words used in the doctrine of Jodo Shinshu (the True Pure Land Sect of Buddhism). The word refers to good acts done in one's past life.
Shuku comes from shukuyo, or past life. However, it isn't limited to the past life before a person was born but also includes all the past until Shukuzen is developed and a person is saved by Amida Buddha.
Shukuzen refers to good acts done to become, as a fate, a believer in Jodo Shinshu.
The words shukuin and shukuen have the same meaning.
In maki no ge (the lower volume) of "Bussetsu Muryoju-kyo" (the Sutra of the Buddha of Immeasurable Life) it is preached as follows:
If a person does not have zenpon (good acts to earn enlightenment of Buddha), he will not be able to listen to this sutra.'
If a person purely obeys commandments, he will be able to listen to righteous teaching.'
If a person has seen Shaka in the past, he will be able to believe this fact.'
A person will devotionally listen and practice the teaching, and will greatly be delighted that a person will start to dance.'
If a person neglects the teaching, he will not be able to believe this principle.'
If a person has seen Buddha in Shukuyo, he will be delighted to listen to the teaching.'
It comes from the statements as above.
That is to say, if it isn't a person who has listened to the Original Vow of Amida Buddha in the past life, that person would not be able to have met the true Buddhism in this life and would not be able to earn the belief (be saved by Amida Buddha).