string of fate
The red string of fate (unmei no akai ito, 運命の赤い糸) is an East Asian belief that every person in the world is connected to another by an invisible red string tied to little finger of the left hand. The two people connected by the red string are destined to be lovers.
It is similar to the Western concept of soulmates. The concept is popularly referenced in anime and manga. Notable examples include Ai Yori Aoshi and Kimagure Orange Road, which frequently reference this concept.