I am the Lord your God, who brought you up out of Egypt. Open wide your mouth and I will fill it (Psalm 81:10 NIV).
God loves to remind us of certain things over and over again. This is because they are very important in themselves and at the same time we are prone to forget them quickly.
The people of Israel quite often demonstrated in their history that they easily forgot God who had brought them out of Egypt.
They did not consider the awesome miracles that God did in the land of Egypt and in the desert wanderings. And to add to the trouble they also went and worshipped other gods.
So God is here reminding the people of Israel that it was He and none else who had brought them out of Egypt.
God was telling them that Egypt was the land of slavery. He had delivered them and set them free. God desired that they would worship Him. Instead the Israelites quite often used the freedom that they got to worship idols and other gods.
It was not because that they found any lack in the blessings of God. But they simply chose to forget their God who brought them out of slavery and cared for them in the desert.
By reminding them again of the deliverance that He had given them God was telling them that He was worthy of their worship. God was also telling them that if they continued to worship Him there would be no lack in His resources to meet their every need.
So He is here found challenging His people to open wide their mouths as far as they could. Their own desires would be the limit; for His supply would be limitless. To the extent they could contain He would fill their mouth with blessings!