A rhyme pattern, or rhyme scheme, indicates which lines of a poem have the same end rhyme.
Depending on the form of poetry, the ending of certain lines may rhyme with each other. In the simplest of rhyme forms, AABB, two lines next to each other will rhyme. Rhyme forms are usually referred to by using letters for each set. In poems that have ABAB schemes, the first line (A) rhymes with the third line, while the second line (B) rhymes with the fourth.
Example (ABAB):
This is line one
And this is line two
When it is done
It's clearer to you