千里 [thousand miles] 姻缘 [destined marriage] 一线 [one thread] 牵 [is led]
The “red string [of fate]” is a concept originating from Chinese mythology (and if you come onto this post wailing about how it’s originally from Japan, you are wrong and need to recognise the fact that just because it’s where you were introduced to the concept doesn’t mean it was originally where it came from, and that this is part of the culture of multiple East Asian countries), wherein the gods tie a red string around the ankles of those who are destined to marry.
The story goes that one day, 月老 (yuè lǎo)，or 月下老人 (yuè xià lǎo rén), the matchmaker god, sat on a cloth sack, reading a book under the moonlight (imagine that). A young, unmarried boy passed by, and, being curious, asked him what he was reading.
月老 told him it was a book of fates, a record of all the marriages that are destined to happen.
The boy then asked what the purpose of the strands of red silk thread in his cloth sack was.
月老 told him they were used to tie together the feet of those who were meant to be married, whether or not the two families had a blood feud, or whether they lived on opposite sides of the country. And once the thread was tied, there was absolutely no way to change it.
He also told the boy that his future wife was the daughter of a vegetable stall vendor a few miles north of where they were. He gave the young boy directions to the market. Once he was there, the boy discovered only a old woman holding an ugly two year old girl.
Being the horrible, superficial brat he was, he didn’t want to marry so ugly a girl, and remembering that the old man had told him there was no way to change who he was tied to, he got a servant to assassinate the girl. The servant, in the dark of night, slashed down towards the girl’s forehead and left without checking that she was dead, unknowingly only wounding the place between the girl’s eyebrows.
Fourteen years later, because of his father’s influence, this boy became an official. The governor of the province liked him very much, and arranged a marriage between this boy and his daughter. His daughter was very beautiful, with the face of a flower and the countenance of the moon (花容月貌), but she always had a flower or an ornament stuck to the space between her eyebrows.
Being curious (ah, continuity), the young man then asked his wife why she always covered that area, and she responded that, in her youth, someone had injured her there, and so she always kept it covered.
Shocked, the young man recounted the story of his murderous assholery, and, realising how fated they were, they fell more in love than ever. I don’t know why, because if my husband told me that when I was two he tried to kill me because he thought I was too ugly to be his wife, I would 休 (divorce) him in two seconds.
Nevertheless, since then, 月老 became known as the god of marriages (and whatever name he might have had in the past, he’s now known as 月下老人, or the old man beneath the moon), and the red string (红线) is a symbol of fate and love.