Three short lines, and one strange, lovely image: an old garden that reads the book of night. The title, "Gloaming," names the hour of twilight, when daylight is fading but darkness hasn't quite settled in. The poem catches that in-between moment and treats the garden almost like a person settling down with a book as evening comes on.
The "yellow bird's eye" is the most puzzling detail. It might be a real bird, or a pale light glowing in the dusk, such as a lamp or a late patch of sky. The poem doesn't say, and that openness is part of its charm. Whatever the eye is, it lets the garden see, and what it sees is night, which the poem pictures as a book slowly opening.
The speaker never states a feeling, but the mood is hushed, patient and watchful. There's a sense of quiet attention, as if the speaker is standing still, noticing the world change, and feeling a gentle respect for it.