Consider the first melodic segment you hear in a song (whether verse or chorus) as A. The next complete melodic segment that you hear that is different than A is B. So, in AABA form, you have two identical melodic segments followed by one different melodic segment, and then back to the original melodic segment. AABA form is used most frequently in slow or mid-tempo ballads, since its 32 bars (four eight-bar sections) would make for a very short song at fast tempos. Examples of AABA songs are: "Just The Way You Are" (Billy Joel), "Don't It Make My Brown Eyes Blue (Richard Leigh), and "Streets of Philadelphia" (Bruce Springsteen). See the sources below for two books that may be of great help to you. They have been my "bibles" for songwriting.
"The Craft and Business of Songwriting" by John Braheny and "The Everything Songwriting Book" by C.J. Watson