The second reading, from the Book of Revelation, gives us biblical imagery of what Heaven is like. St. John the Apostle, the author of this book, saw a vision of the heavenly city. Just like cities have boundaries and entrances and streets, so does the heavenly one.
The last paragraph from this reading tells us something that is quite remarkable. There is no need for the sun or the moon for illumination in Heaven because the lamp of the city is the Lamb. We know that Jesus is the light of the world. St. John, in his Gospel, uses this image more than once. Jesus is the light that has shone in the darkness.  This will reach its fullness in Heaven, as the light of the world, Jesus Christ, who is the Lamb of God, will be the light that illuminates everything in Heaven.
We should take this at the literal or physical level. I think that Jesus will be providing physical light to all the inhabitants in Heaven, at least for the human ones. The angelic inhabitants do not need physical light, since they are spiritual beings and do not have physical bodies. Jesus will provide light for us to illuminate our eyes. We will be able to see the beauty of Heaven by the light of the Lamb.
Since we are both physical and spiritual beings, the light of the Lamb will also provide spiritual light. Our souls have two powers or abilities, and they are the intellect and will. These will be illuminated by the spiritual light of the Lamb, especially our intellects. Everything that we see and know by the eyes of our minds will be known through the Lamb. Our intellects will be elevated. We will know more truths, since we will be beholding the Beatific Vision. This phrase is used to describe the reality that all in Heaven are perpetually before the Triune God.
The Lamb will be mediator of this, as Jesus tells us that come to the Father through Him. Jesus, in His humanity, is the bridge between mankind and God. He is the mediator between what is human and what is divine, since both exist in Him. Jesus has both a human nature and a divine nature. This is why the lamp of the City of Heaven is the Lamb. Jesus has won the victory. He is the conqueror. He is the light of the world. At the name of Jesus all beings will bow.
What we participate in through signs, the sacraments in general and the Mass and Eucharist in particular, will come to its fulfillment in the next life. This is what we have to look forward to. So may this help to motivate us to live for Him now, not tomorrow or next week, but today. May we never assume that we will go to Heaven, but begin living the heavenly life in the Church through frequenting the sacraments, which gives us tastes of Heaven.