Theory:

The pattern of rhymes at the ending of each line in a poem is called a rhyme scheme. Letters (A,B,C...) are usually used to express which lines rhyme. Verses that are designated with the same letter are said to rhyme with each other. It is also known as an arrangement of rhymes in a stanza or a poem.
Example:
For an easier understanding of the concept, let us take a famous nursery rhyme written by Jane Taylor as an example.
 
Twinkle twinkle little star,
How I wonder what you are.
Up above the world so high
Like a diamond in the sky.
 
We see that the first two lines rhyme with each other (star-are); the second two lines rhyme with each other (high-sky). Let us name each line as \(A\), \(B\) depending on the words that rhyme with each other.
 
Twinkle twinkle little star,
A
How I wonder what you are. A
Up above the world so high,B
Like a diamond in the sky. B
 
It can thus be seen that this poem follows \(AABB\) pattern.
Rhyme scheme of the poem "A Slumber did my Spirit Seal":
A slumber did my spirit seal\(A\)
I had no human fears\(B\)
She seemed a thing that could not feel\(A\)
The touch of earthly years\(B\)
No motion has she now, no force\(C\)
She neither hears nor sees\(D\)
Rolled round in earth’s diurnal course\(C\)
With rocks and stones and trees\(D\)
It can thus be seen that this poem follows \(ABABCDCD\) pattern.