"Therefore when the Lord your God has given you rest from all your enemies on every hand, in the land that the Lord your God is giving you as an inheritance to possess, you shall blot out the remembrance of Amalek from under heaven; do not forget" (Deuteronomy 25:19).
Amalek was the ruler of the Amalekites, a fierce tribe that would fight to keep the land the Israelites intended for themselves. You can read about the battle in Exodus 17:8.
Beneath this factual level, what this particular verse suggests to me is that, in our personal spiritual development, we must not relax when the appearances of a challenge have been overcome. There is always a fear-based error thought in our ego mind that made the challenge; it is by identifying and dissolving that error thought in the greater energy of our true Spirit.