Complex sentence: Costello explains that she is seeing the future (her "visions" are not flashbacks, but flashforwards), and that they have come to help humanity, for in 3,000 years they will need humanity's help in return.
Split into simpler sentences: Costello explains that she is seeing the future (her "visions" are not flashbacks, but flashforwards). Costello adds that they have come to help humanity, because in 3,000 years they will need humanity's help in return.