This passage occurs in the Narrative Lectionary Year 4, Advent cycle. It also appears in the Easter Vigil and at other points in the RCL.
Summary: This passage picks up with a number of great Advent themes: Hope, repentance, waiting for God's fulfillment. For those who still remember the days of Advent III being about joy, there is much joy in this passage, including God's delight, even pleasure, in his word being fulfilled.
To make it really simple: Part of the joy of Christmas, part of why it gives us such peace, joy and hope is because it is so sensory! Light a candle and sing a song!
חסד (kased, "loving-kindness," Isaiah 55:3) This word is a tough one to translate. The idea is steadfast and undeserved love. I think those who have been walking through the narrative lectionary may be able to communicate finally what is going on: God is faithful through every step and every level of human disobedience. This is not simply a God who hangs in there, but hangs in there four centuries finally only to hang himself for the sake of humanity.
נפש (nephish, "soul", Isaiah 55:2-3) For those of us living in a world formed by Greek ideas, we hear soul and we think the wispy part of our lives that floats up to heaven. But actually, the word "soul" here in Hebrew means living being. This is clearly identified by the context here that includes eating, drinking and listening. The soul, even if one wants to move into Greek territory of a distinct soul from body, is not separate but intimately connected.
נתנ (nathan, "give", 55:4) The prince is not made, but the prince is given. I think this is an important reminder about the nature of leadership. Leaders aren't made, they are given by God.
מצוה (mitzvah, "command", 55:4) In this case, the leader is given with a particular purpose, to command them. He is not a commander in the grammar here; he will command the people. But this word command is related to God's command for the people. This seems to harken back to Isaiah 2, then when all the nations of the Lord stream to God and learn his law.
תפצ (kephitz, "delight", 55:11) God does not simply intend for his word to be fulfilled but to delight him. I think this is a great way to end the passage, with God having great joy, even pleasure and delight in his word.
Key word we had two weeks back (in Jeremiah 37)
מחשבה ("makhashaba"; "thoughts, devices, or plans"; 29:11): I am always worried about the word plan in the Bible. It can quickly make both humans and God into a fatalistic machine. The word translated plan here can be mean plan, but it can also mean thought or device/plot:
NRS Psalm 92:5 How great are your works, O LORD! Your thoughts are very deep!
NRS Isaiah 55:8 For my thoughts are not your thoughts, nor are your ways my ways, says the LORD.
NRS Proverbs 16:3 Commit your work to the LORD, and your plans will be established.
NRS Lamentations 3:61 You have heard their taunts, O LORD, all their plots against me.
As the TWOT indicates: "The basic idea of the word is the employment of the mind in thinking activity. Reference is not so much to "understanding" (cf. bi^n), but to the creating of new ideas." What further attests to this is that the LXX translates this as λογισμος, which means thought or reason. I do not disgree that God has plans for the people. In this case, Jeremiah has very clear prophecies about 70 years. But the OT word for plan in many cases means something softer than "calculated plan" and more like "creative and reasoned thoughts" that have as their end peace and hope.