Rhyme scheme is a pattern form of rhyme that comes at the end of each verse or line of poetry or song. It is identified by end letter of each lines of a poem. If a poem does not have a rhyme scheme then it is considered a free verse poem.
For example is-
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)
Here the rhyme scheme is AABB. “Star and are” and “high and sky “sound same at the end.