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 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 highB
Like a diamond in the sky.B
It can thus be seen that this poem follows  AABB  pattern.
Rhyme scheme of the poem "Little Sarah":
 
Little Sarah she stood by her grandmother’s bed,A
“And what shall I get for your breakfast?” she said;A
“You shall get me a Johnny-cake: quickly go make it,B
In one minute mix, and in two minutes bake it.”B
So Sarah she went to the closet to seeC
If yet any meal in the barrel might be.C
The barrel had long time been empty as wind;A
Not a speck of the bright yellow meal could she find.A
 
But grandmother’s Johnny-cake-still she must make it,B
In one minute mix, and in two minutes bake it.B
 
She ran to the shop; but the shopkeeper said,A
“I have none—you must go to the miller, fair maid;A
“For he has a mill, and he’ll put the corn in it,B
And grind you some nice yellow meal in a minute;B
 
But run, or the Johnny-cake, how will you make it,B
In one minute mix, and in two minutes bake it?”B
Then Sarah she ran every step of the way;D
But the miller said, “No, I have no meal to-day;D
Run, quick, to the cornfield, just over the hill,E
And if any be there, you may fetch it to mill.E
 
Run, run, or the Johnny-cake, how will you make it,B
In one minute mix, and in two minutes bake it.B
 
She ran to the cornfield-the corn had not grown,F
Though the sun in the blue sky all pleasantly shone .F
“Pretty sun,” cried the maiden, “please make the corn grow.”G
“Pretty maid,” the sun answered, “I cannot do so.”G
 
“Then grandmother’s Johnny-cake—how shall I make it,B
In one minute mix, and in two minutes bake it.B
 
Then Sarah looked round, and she saw what was wanted;A
 The corn could not grow, for no corn had been planted.A
 
She asked of the farmer to sow her some grain,F
But the farmer he laughed till his sides ached again.F
“Ho! ho! for the Johnny-cake-how can you make it,B
In one minute mix, and in two minutes bake it?”B
 
The farmer he laughed, and he laughed out aloud,-A
“And how can I plant till the earth has been ploughed?A
 
Run, run to the ploughman, and bring him with speed;A
He’ll plough up the ground, and I’ll fill it with seed.”A
 
Away, then, ran Sarah, still hoping to make it,B
In one minute mix, and in two minutes bake it.B
 
The ploughman he ploughed, and the grain it was sown,F
And the sun shed his rays till the corn was all grown;F
It was ground at the mill, and again in her bedA
These words to poor Sarah the grandmother said:A
“You shall get me a Johnny-cake-quickly go make it,B
In one minute mix, and in two minutes bake it.”B
The rhyme scheme for "Little Sarah" is AABBCCAABBAABBDDEEBBFFGGBBAAFFBBAAAABBFFAABB.
Reference:
State Council of Educational Research and Training (2018). Term-1 English Standard-9. Little Sarah by Anna Maria Wells (pp. 206-208). Published by the Tamil Nadu Textbook and Educational Services Corporation.