Picture taken from this site.
A Twin flame is the most important someone who could be ever met.
For people believing in reincarnation, twin flames are souls meant to reincarnate many times in order to finally meet and join together in the end of their ways before ascending a higher level of existence.
For example here you could read more about it.
As far as oneiric personalism is concerned (meant to be a metaphysical, not an esoteric concept): I dream I am a person – Marcin Dolecki [here readers could put their names], explaining this issue in other words: M. Dolecki is merely an image of my true, absolute Self in the mirror of nothinghess. Other persons are also images in that peculiar mirror. Some of these images could be especially similar to each other therefore there should be a person who is the most similar to me; this person would be extremely attractive for me (perhaps also sexually).
I exclude the possiblility that there could be more than one such person because similarity between beings is not “quantified”. This person could be called a twin flame. All what is real in that person, is in fact my true Self.
I think it would be nice for everyone to meet a twin flame, but it is not necessary for spiritual growth. From the pure statistical point of view, it would be an improbable event but perhaps there are some “cosmic mechanisms” in action which enable such meeting.
Only turning towards the Absolute is unconditionally necessary for spiritual development, nevertheless, interaction with a twin flame might be of value as far as he/she is a unique image of the Absolute.