The First Impact was the event when a meteor came to Earth landing on Peru.
It caused a great surge of energy including "white nights" (lightened nights).
The meteor contained EVA on it, and a scientist group was sent by United Nations to investigate.
It is also said some time in the future there will be a so-called "Second Impact", but the meaning of that is unknown but for some members of the SHINE sect including Brazel.
WARNING: Fan theories start there
Second Impact could refer to a massive EVA unleash, creating an infection all around the world, to which only a few EVA Hybrids could survive.