The concept of soulmates is based on the cultural idea that two people are so compatible with each other that their "souls" are bonded together. It often states that every single person has their one soulmate in the universe. Remarkably most people find their "soulmate" handily in the same town as them and not living in an isolated tribe in Papua New Guinea. The idea of a soulmate is often cited in sync with the idea that the world is predestined and therefore your mate is predestined as well. It also comes up in discussion around reincarnation and the idea that people have met in their past life.
There is no current evidence of a soul, let alone a soulmate. Divorce rates hover around 50% in the Western World suggesting humans are pretty bad at finding their soulmates. As far as science can tell "love" is a hormonal response to physical and emotional stimuli.
Happy Valentine's Day and don't forget to tell the human you have decided to pairbond with that your internal hormonal response currently creates warm feelings towards them.