A rhyme scheme is the pattern of rhyming lines in a rhyming poem or in lyrics for music. It is usually referred to by using letters to indicate which lines rhyme.
For example "abab" indicates a four-line stanza in which the first and third lines rhyme, as do the second and fourth. Here is an example of this rhyme scheme from To Anthea, Who May Command Him Any Thing by Robert Herrick:
Bid me to weep, and I will weep,
While I have eyes to see;
And having none, yet I will keep
A heart to weep for thee.