A footstool means a low, small piece of furniture that you can put your feet on when you are sitting. In the Bible, the footstool is variously likened.

1 Chronicles 28:2
King David rose to his feet and said: "Listen to me, my brothers and my people. I had it in my heart to build a house as a place of rest for the ark of the covenant of the LORD, for the footstool of our God, and I made plans to build it.

The ark of the covenant is compared to the footstool of our God.
And again, the whole earth is likened to God's footstool.

Isaiah 66: 1 This is what the LORD says:
"Heaven is my throne, and the earth is my footstool.

From the Almighty God's point of view, the earth is counted for nothing, so it is likened to the footstool. Also, the devil which is to be defeated by Christ is likened to God's footstool.

Psalms 110: 1,4
The LORD says to my Lord:
"Sit at my right hand until I make your enemies a footstool for your feet."... The LORD has sworn and will not change his mind:
"You are a priest forever, in the order of Melchizedek."

The above is the prophecy about Christ who came to the earth as the High priest in the order of Melchizedek. That the devil which is God's enemy would be God's footstool shows that God will win eternal victory in spiritual battle with the devil. And the outcome of the battle is decided to be God's victory.

Revelation 17:14
They will make war against the Lamb, but the Lamb will overcome them because he is Lord of lords and King of kings--and with him will be his called, chosen and faithful followers."