Nothing is so beautiful as spring – (line 1)
Notice how the speaker repeats the word "spring" even though it was just given in the title. It seems offhand, but it also makes us think a little harder about all the connotations of spring, particularly in the Christian religion: renewal and re-creation, Easter, and the resurrection of Christ.
When weeds, in wheels, shoot long and lovely and lush; (line 2)
This line tells us of the speaker's deep delight in the shapes and growths of spring.
it strikes like lightnings to hear him sing (line 5)
Our speaker has a keen eye and ear, and an ability to be awed by things that many of us might be too distracted to even notice. (If you want to hear the thrush's song, click here.)