The speaker is stuck indoors while the first rain arrives, and everything outside seems to be inviting them out. Clouds swing like curtains, trees speak in cheerful voices, and the winds are busy with what the speaker calls "many works." The speaker's feeling is eager longing, close to impatience. The repeated plea to "open the door" shows how badly they want to join in.
The most interesting idea is the trees, described as "happy trees... who have no in or out." Unlike people, they don't live behind walls, so they can celebrate freely. The door becomes more than a wooden one; it suggests the line people draw between themselves and nature. By the end, clouds, trees and wind together "raise joy from depth of earth," as if the whole world is waking up. The poem's last words, "Let me go...", trail off, leaving a mood of joyful, almost childlike excitement. The slightly rough phrasing adds to that spontaneous, breathless feel.