A rhyme scheme is the pattern of end rhymes in a poem or stanza. Poets usually mark the pattern with letters, giving matching letters to lines that rhyme and new letters to lines that do not. For example, abab means the first and third lines rhyme, and the second and fourth lines rhyme.
Example
Bright falls the dusk on the quiet street,
A lamplit window glows with gold;
The shadows gather at my feet,
And evening folds the day in cold.
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.