The concept refers to the way the various sections of a song (normally 3: Verse, Chorus and Bridge, or A, B and C) are positioned in relation to each other. A and B are re-occurring elements, while C tends to happen only once. Each one of these section, for variety’s sake, is supposed to differ harmonically and/or melodically from the other ones and carry a certain “emotional weight” — it can be calm, transitioning, or tense. A classic pop song has a verse repeated twice that slowly builds in tension towards a chorus — the emotional peak. This AAB succession is often also repeated twice, and then followed by the bridge which has the function of adding variety and “resetting” our ears and brains to prepare them for the gran finale of a double chorus: AABA(A)BCBB. There are hundreds of variations on this example.
Bridges are a crucial element, but are often “forgotten” by emerging bands. The Beatles knew something about it, going through their Complete Chord Songbook might be the fix you need to get your song structures right.