sukideen:

The red string of fate, is an East Asian belief originating from Chinese legend and is also used in Japanese legend. According to this myth, the gods tie an invisible red string around the fingers of those that are destined to meet each other in a certain situation or help each other in a certain way. According to Chinese legend, the deity in charge of “the red thread” is believed to be Yuè Xià Lǎo (月下老, often abbreviated to “Yuèlǎo” [月老]), the old lunar matchmaker god who is also in charge of marriages.

The two people connected by the red thread are destined lovers, regardless of time, place, or circumstances. This magical cord may stretch or tangle, but never break. This myth is similar to the Western concept of soulmates or a twin flame.