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 "For Anne Gregory":
 Never shall a young man $$A$$ Thrown into despair B By those great honey-coloured, C Ramparts at your ear, B Love you for yourself alone D And not your yellow hair B

 “But I can get a hair-dye $$A$$ And set such colour there, B Brown, or black, or carrot, C That young men in despair B May love me for myself alone D And not my yellow hair.” B

 “I heard an old religious man $$A$$ But yesternight declare B That he had found a text to prove C That only God, my dear, B Could love you for yourself alone D And not your yellow hair.” B

It can thus be seen that this poem follows $$ABCBDB$$ pattern.