Isaiah 55 begins with a rousing encouragement - come for water, all who are thirsty; though you have no money, come, buy grain and eat; come, buy wine and milk, not for money, not for a price. This passage is a resounding reminder as to how different God's ways are from ours. Later in the chapter the prophet says, passing on God's message - for my thoughts are not your thoughts, nor are your ways my ways. As I often say in preaching, God turns things upside down!
The reminder is that God's ways are higher than ours. We need to look up to God. The images of the chapter - watering the earth, giving seed for sowing and bread for eating - are reminders of the abundance of God's gifts. [Am I ready to receive God's gifts?] We often talk - rightly - about the importance of giving, but receiving is also important!
The image of the mountains and the hills shouting for joy and the trees clapping their hands is crazy - but that craziness equals the love of God! It is all about transformation.