Below is the text of the poem. Line 2 says it is is "fresh". Lines 3 and 4 talk about how it blooms and is pleasing, even though it is alone in a damp, deserted spot. Lines 7-8 talk about it providing cool shade for the birds. Lines 9-12 point out that the plant is wise and beautiful, and it's beauty is sufficient reason for it to exist. Below is listed an enotes summary which might help you too.
IN May, when sea-winds pierced our solitudes,I found the fresh Rhodora in the woods, Spreading its leafless blooms in a damp nook, To please the desert and the sluggish brook. The purple petals, fallen in the pool, 5 Made the black water with their beauty gay; Here might the red-bird come his plumes to cool, And court the flower that cheapens his array. Rhodora! if the sages ask thee why This charm is wasted on the earth and sky, 10 Tell them, dear, that if eyes were made for seeing, Then Beauty is its own excuse for being: Why thou wert there, O rival of the rose! I never thought to ask, I never knew: But, in my simple ignorance, suppose 15 The self-same Power that brought me there brought you.
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.