What does it mean to say that a limerick is in the form aabba?

"AABBA" is a way of describing the rhyme scheme of a limerick. It means:

* Line 1 rhymes with line 2 and line 5. (These are represented by the "A" in the scheme)

* Line 3 rhymes with line 4. (These are represented by the "B" in the scheme)

For example, here's a simple limerick:

There once was a fellow named Dave,

Who lived in a house by the wave,

He loved to eat fish,

And loved to do dish,

And lived a life free from crave.

Notice how:

* Dave rhymes with wave and crave

* lived rhymes with fish and dish

This is the classic limerick structure.