It is called Rhyme Scheme ( type: enclose rhyme).
Rhyme is the repetition of sounds in a word used for example at the end of lines of poetry, within a word, in songs, etc.
So as a consequence, Rhyme Scheme is the pattern of rhymes, for example at the end of a poem. They are representing by using letters of the alphabet so that each rhyme in a poem that rhyme with other has assigned a letter (starting with the letter 'A').
Enclose rhyme or enclosing rhyme is the rhyme scheme ABBA. For example:
"Love had us mesmerized under its magic spell A
Our youth's enchanted vision of what life would be B
Innocent intentions that let our lives feel free B
Enraptured in each other's arms as twilight fell." A
