Lines 3 and 5 tell us something about the Rhodora.

I found the fresh Rhodora in the woods,
Spreading its leafless blooms in a damp nook,   (3)
To please the desert and the sluggish brook.
The purple petals fallen in the pool,                (5)
Made the black water with their beauty gay;

In line 3, we learn the Rhodora is a shrub that flowers before it gets its leaves. Its petals are purple, and they stand in sharp contrast to their surroundings of the stagnant pool. Other than those lines, Emerson gives no other description of the Rhodora except to say its beauty shames that of the redbird and rivals the rose.

Notice also the line in italics at the beginning of the poem. "Whence" does not mean when, but is asking where the flower comes from, implying whether the Rhodora has a purpose in its existence. This question is asked by a companion of the speaker of the poem. In the end, Emerson says God made the Rhodora just like He made man.  As such, the Rhodora has the right to ask man what his purpose is. Emerson is telling us not to put man above nature.