"A Dream Within a Dream" poem opens with a kiss, which the speaker gives to an unspecified addressee. It's a kiss "upon the brow," so it isn't necessarily a romantic kiss—but that does seem the most likely scenario given what follows. In other words, the speaker is probably talking to a lover. The gentle consonance and assonance of the line convey the affection of the kiss:
Take this kiss upon the brow!
It's an interesting opening to a poem because it happens without any attempt to establish a context for the kiss—it just happens. There is a sense of urgency, then, and perhaps even desperation. This is confirmed by lines 2 and 3, which reveal that the speaker is "parting" from the addressee—and in fact is doing so right "now." These lines suggest that the rest of the stanza will be the speaker's words of goodbye—the words that the speaker feels most need to be said given that these two people are having to go their separate ways (though, again, the circumstances of this are not clear).
Following the dash at the end of line 3, the speaker offers up some parting words—which themselves are a response to something that the addressee has evidently said before. "You are not wrong," says the speaker, in other words confirming that the addressee was correct to say the following: that the speaker's "days have been a dream." It's worth taking note of the vagueness of this scenario—there is no sense of who the speaker is meant to be, nor the identity of the addressee. It's not even certain whether the addressee is one individual, or many. However, this ambiguity feeds into the poem's central idea: that reality is one some level fundamentally illusory or unknowable. To that end, there is something child-like and wondrous about the alliteration across lines 4 and 5:
You are not wrong, who deem
That my days have been a dream;
These /d/ sounds are playful, almost like trickery—like the kind of stuff the Mad Hatter might speak in Alice in Wonderland. At this point, it's also not known if the idea put forward in these lines is meant positively (viewing life as a kind of dream come true) or in the sense of life being a kind of illusion. There's also the possibility of a sort of in between interpretation—that this means the speaker has been living in a sort of fantasy world, not really understanding the reality of the relationship with this unknown addressee.