How would you explain "A Birthday" by Christina Rossetti?

"A Birthday," for Christina Rossetti's narrator, is not a traditional birthday, but rather the day that marks her experience of first love. The effusiveness of this feeling is addressed in the first line, "My heart is like a singing bird." A bird sings to communicate. The simile expresses the sense in which the narrator is in communion with her love. 

The first stanza uses anaphora, or repetition of the first words of each line. "My heart" is repeated and compared to aspects of nature using similes. The speaker's heart is not only a singing bird, but also "an apple-tree" and "a rainbow shell." The comparisons are connected to images of fertility and abundance:

My heart is like a singing bird

Whose nest is in a watered shoot:

My heart is like an apple-tree

Whose boughs are bent with thickset fruit;

My heart is like a rainbow shell

That paddles in a halcyon sea;

"A watered shoot" signifies nourishment, while the "apple-tree / Whose boughs are bent with thickset fruit" appears to be the result of that nourishment—ripeness and lush fecundity. On the other hand, "a rainbow shell" is, unlike an apple tree, something rare and elusive. It is varicolored, which complements the complex and shifting nature of love. Here, it "paddles in a halcyon sea." Its active movement indicates that it is alive, while "halcyon" indicates that light and warmth glow all around it.

The first stanza is replete with resplendent imagery. There is a springing iambic rhythm, and every second line is in perfect rhyme.

The second stanza draws us out of the natural world and into the human social world of festive celebration. Rossetti maintains the rhyme scheme, yet now each line begins with a verb in the imperative tense.

Elements of the natural world now become artifice, luxurious adornments for the dais, which symbolize her sense of being elevated or honored. She is celebrating "the birthday of [her] life," marked by the coming of her love. In other words, life does not begin until one first knows love in all its lush beauty.