The prefix of again — return, repetition, renewal; meaning's way of coming back.
Latin re- (again, back), a prefix of disputed further origin, possibly related to a form appearing as red- before vowels; whatever lies beneath it, the prefix's own sense — return, repetition, renewal — has stayed remarkably stable across two thousand years of borrowing.
The prefix of again — return, repetition, renewal riding on whatever root it is attached to. It is meaning's own way of saying a thing has been here before.