Theia is, or rather was a planet that hypothetically collides with earth some 4,5 billion years ago. The collision would have been the source of earth's moon when the debris from the collision formed a celestial object orbiting our planet. It is estimated that 70-90 percent of the matter of the moon is from Theia. Earth also consist of some part of the Theia which would explain why earth's core is bigger than expected.
Theia was supposedly formed in the outer solar system and for some reason drifted put of it's orbit towards the inner solar system until it collided with the proto-earth. Originating from outer solar system would explain most of the water on earth as well.
Theia has been estimated being around the size of Mars and it would have had the requirements similar to earth for the formation of life would it have been on a stable orbit around the same zone as earth (the Goldilocks zone).
There are other theories for many of the things that makes our planet unique in many ways, but the Theia hypothesis would explain many of them with a single event even if such a cosmic incident would bee highly improbable.