What does ABAB mean in Poetry? Can you give me an example of a famous ABAB poem?
ABAB refers to the rhyme scheme of a poem. A rhyme scheme is a pattern of sounds that repeat at the end of the line or stanza of a poem.  So in an ABAB pattern, the first and third lines rhyme with each other and the second and fourth lines rhyme.  For example: 

I have a rat  (A)
I have a mouse  (B)
I have a cat  (A)
I have a house  (B)

Some examples of famous ABAB poems are:
"Elegy Written in a Country Churchyard" - Thomas Gray
"Neither Out Far Nor in Deep" - Robert Frost
"Ballade of Modest Confession" - Hilaire Belloc
"Shall I Compare Thee to a Summer's Day" (Sonnet 18) - William Shakespeare