In the first stanza of "Spring," Hopkins conveys the intense energy that emerges during the spring season. That energy is conveyed by Hopkins's description of the song of the thrush, a small songbird which is known for being loud.
The quote about the thrush is as follows:
Thrush's eggs look little low heavens, and thrush
Through the echoing timber does so rinse and wring
The ear, it strikes like lightnings to hear him sing.
Rather than writing calm verses about birds tweeting sweet and lovely tunes in spring, Hopkins strikes and startles us with the stunning energy of the thrush's song. This bird's song is not placid but painful in its intensity. It "rinses" and "wrings" the ear, images that convey doing laundry. As the process of rinsing dirty laundry with hot water and then wringing it out cleans and purifies it, so the ear, Hopkins is saying, is painfully cleaned and purified by the sound of the thrush. The idea that the sound of the song is painful (but at the same time illuminating) is reiterated with the image that it strikes the ear like lightning bolts.
What Hopkins is trying to communicate through these startling images becomes clear in the next stanza. In it, he explains that what he calls the "juice" and "joy" of spring is a reminder of the garden of Eden before the fall of man. We should experience it in its stunning, vibrant newness before it becomes "cloying" or sickly sweet. In other words, we shouldn't let weak and sentimental images of spring prevent us from feeling its power. This is the power of God still echoing through the earth even after the fall.