What does it mean the earth is God's footstool?

Hi I was asked from a friend what does God mean by the earth being his foot stool?
Thanks And God Bless

Thus says the LORD: The heavens are my throne, the earth, my footstool.
What house can you build for me? Where is the place of my rest?

My hand made all these things when all of them came to be—oracle of the LORD.
This is the one whom I approve: the afflicted one, crushed in spirit, who trembles at my word.

The one slaughtering an ox, striking a man, sacrificing a lamb, breaking a dog’s neck,
Making an offering of pig’s blood, burning incense, honoring an idol— These have chosen their own ways, and taken pleasure in their own abominations.

- Isaiah 66:1-3

God is reminding us that we cannot please God merely by building something large and beautiful since He is greater than these created things. Yes, Israel had the temple but they were not truly fulfilling the covenant. This is reflected in one of the weekday prefaces to the Eucharistic Prayers which states: “You have no need of our praise,
yet our desire to thank You is itself Your gift. Our prayer of thanksgiving adds nothing to Your greatness, but makes us grow in Your grace.”
We cannot get caught up in thinking we have a one-to-one relationship with God (i.e. I do something for you, you do something for me). Anything that we attempt to give back to God is something that He already gave to us. Therefore the most important things are our intentions, our faith, or trust in God.

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