The poem "The Lake Isle of Innisfree" tells of a daydream the poet has while he walks the streets of a city, as he says: "while I stand on the roadway, or on the pavements grey." He longs for a simple life far from the noise and filth all around him.
The poet's daydream is quite vivid. He will build a small cabin of sticks and rods, plant beans, and keep honey bees. He will hear only soothing natural sounds such as the humming of the bees, the chirping of crickets, and the lapping of waves on the lakeshore. Instead of vistas of buildings, traffic, and pedestrians he will enjoy glowing noontimes and glimmering evenings on the peaceful isle.
Although he walks city streets, his thoughts are full of this peaceful place that he has imagined and the quality of life he would have there. He feels unrequited longing, but also having this vision, even if he cannot fulfill it, also probably gives him a feeling of peacefulness and serenity.
The lake isle of Innisfree is a real place, and Yeats spent childhood summers there. In his autobiography he describes having a similar experience to the one he writes of in this poem while walking along a street in London. That's one reason why the thoughts and feelings expressed in this poem are so vivid.