From the glossary:

Footstool is a tough place for an enemy to be. "The Lord says to my Lord, ‘Sit at my right hand, until I make your enemies your footstool.’" (Ps 110:1) Jesus referred to this several times, in Matthew 22:44, Mark 12:36, Luke 20:43, and others. The full meaning of making your enemies your footstool is "..Joshua called for all the men of war who went with him, ‘Come near, put your feet upon the necks of these kings.’ And they came near, and put their feet upon their necks. And Joshua said to them, ‘Fear not, nor be dismayed, be strong and of good courage: for thus shall the Lord do to all your enemies against whom you fight.’ And afterwards Joshua smote them, and slew them, and hanged them on five trees:"

(Josh 10:24) When he smote them he cut off their heads, completely removing the enemy - the meaning of "make your enemies your footstool."