In just five short lines, the speaker describes turning toward a fir tree and finding something unexpected: a wish to be quiet, the way a bride might be quiet. The opening claim is a little puzzling. The fir is "never seen in draft," as if it always appears finished, never rough or half-formed, and it stands "among the greatest company," perhaps meaning it holds its own beside the grandest things in nature. The tree is called "her," which makes it feel like a person the speaker can approach and confide in.
What the speaker feels seems to be admiration shading into hush. Standing in a garden, they don't talk about the tree so much as take on its calm. The bride image suggests a quiet that is expectant and solemn rather than empty. The mood is gentle, reverent and slow, like the pause you make when something steady and complete is in front of you. The poem leaves the exact meaning open, but the feeling comes through clearly.