The poem opens with the speaker recounting a dream they once had. The speaker personifies the dream as a kind of mysterious weaver, saying that it "w[ove] a shade / O'er [the speaker's] angel-guarded bed."
This moment can be taken literally, suggesting the "shad[y]" darkness of the night. But here, it might be more fitting to think of this "shade" as a kind of enchantment. In other words, a dream cast a spell on the speaker, so that instead of feeling safe and protected in their bed, the speaker thinks they are lying on "grass" in the darkness of night, witnessing the distraught wanderings of a lost "emmet," or ant.
The fact that the speaker is dreaming of the ant (rather than just observing an ant in the real world) suggests that the ant is of some personal significance to the speaker; the speaker, it seems, relates to this little lost ant, wandering in the darkness.
The poem immediately contrasts "shade," or darkness, with the speaker's "angel-guarded bed," suggesting that these two things are at odds. In the dream, it seems, the sleeper is afraid of not being able to find a way back to the waking, benevolent world of their bedroom—or perhaps they're just feeling lost and bewildered in their waking life.
Either way, this dream will work rather like a fable, a morality tale with a cast of anthropomorphized animals. This will be a tale of being lost and found, lonely and protected.