Remember what Amalek did unto thee by the way, when ye were come forth out of Egypt;

Remember what the Amalekites did to you along your way from Egypt,

How he met thee by the way, and smote the hindmost of thee, even all that were feeble behind thee, when thou wast faint and weary; and he feared not God.

how they met you on your journey when you were tired and weary, and they attacked all your stragglers; they had no fear of God.

Therefore it shall be, when the LORD thy God hath given thee rest from all thine enemies round about, in the land which the LORD thy God giveth thee for an inheritance to possess it, that thou shalt blot out the remembrance of Amalek from under heaven; thou shalt not forget it.

When the LORD your God gives you rest from the enemies around you in the land that He is giving you to possess as an inheritance, you are to blot out the memory of Amalek from under heaven. Do not forget!