The Fall of Man happened soon after the Creation of mankind. The first man and woman had been created perfect and were put in the Garden of Eden to guard and keep it for God. However, there was a test of their character placed deep within the garden: the Tree of Knowledge (of Good and Evil). The first couple, tempted by Satan in the guise of a serpent, failed the test.
The result was catastrophic, affecting all of creation as well as the first parents of mankind. Within less than two thousand years things had gotten so bad that God had to judge all of mankind in the Great Flood. The fall was into sin and away from God. The way back to God would be through a "Savior" -- a person descended from Adam and Eve but especially appointed by the Creator.