The term, 'In-Yeon' is one of the words which I haven't found the proper substitutes for in English and in Spanish.
A simple example can be helpful to your understanding the meaning easily. There are two people walking on the street. When their clothes get brushed with each other's, they say or think that there must have been something between two of them in the past that they don't know. The 'something' is called 'In-Yeon'.
More concretely, 'In-Yeons' can be particular relationships between people or things formed during the past lives. 'In-Yeons' can also be the specific causes of certain results which happened in the past.
The word, 'In-Yeon' will appear here and there in my blog frequently so you must have the obvious understanding of it. If you don't have sufficient understanding or if you have any inquiries, please let me know through comments here or through e-mails (firstname.lastname@example.org).