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

It simply means that the first two lines and the last line end with the same rhyme, and the third and fourth lines have a different rhyme to the others.

For example:

An Anglican curate in want (a)

Of a secondhand portable font (a)

Would exchange, for the same (b)

A portrait, in frame, (b)

Of The Bishop Elect of Vermont (a)