A bridge (part B of a song in AABA form) should have these elements:
*It should be contrastive in some way with the A sections. A different key, a different rhythmic pattern? Some difference, let's say.
*It should follow logically from the A section too. It is not some unrelated piece of music thrown in there. It might follow logically in harmonic, melodic, or rhythmic ways, or in some combination.
*It has to lead back in to the A section, making you want to return there happily.
For a while I've been working on a song whose bridge will not work for me very well. It is fine but I can't sew it back at the end to the A section.