In the astrophysics sense or in the woo sense?

It's a being from another star, constellation, or time dimension that has reincarnated into a human family to help humans achieve a higher dimension.

Urban dictionary:
"Starseed Is a person who is spiritually aware, having a strong connection to the divine creator. Starseeds are said to be old soul sent back to earth to transform the world into the heaven on earth predicted after the end of times. Such persons may display spiritual gifts of clairvoyance, clairaudence or clairsentences. Being able to read minds, others emotions or see future events."