Psalm 81:10 I am the LORD thy God, Which brought thee out of the land of Egypt: Open thy mouth wide, and I will fill it.
The God of the Bible reminded His people of His power. He had delivered them from the greatest nation in the world. He had crushed the global power. God had shown Himself to be God over all the false gods of Egypt. He now calls on His people to realize that He hears and answers prayer. They can ask for big things, and He will answer.
They were to open their mouths wide. They were, in other words, to ask for big stuff. They did not need to come timidly and doubting but in full assurance and confidence. He is God. There is no other. He hears and answers prayer. They can ask confidently for big things.
God invites you to pray. If you are saved, you are His child. If you are His child, He calls on you to believe Him and ask Him.