Much like a physical bridge, it’s the way we get from one place to another. There is often melodic contrast, a mood shift, and either new information or a new perspective.
A bridge has a fairly simple directive to fulfill: be different, create a contrast, and make the listener long to go back to another section. Just like a real bridge, it takes you from one place to another. It connects song sections and moves to a different level of information than the other sections. It’s an opportunity for a new perspective. Sometimes in the form of a question. Musically, it’s a chance to offer the listener something they haven’t heard before.
A bridge can focus on any kind of material: specific, abstract, philosophical, internal landscape, external landscape, emotional reaction, metaphorical interpretation, etc. But it should not move the story forward like the verse does. It is not the bigger picture, like the chorus.
A bridge in a verse/chorus song usually occurs after the second chorus, often leading to another chorus, though it sometimes leads into another verse. In verse/refrain songs, the bridge usually occurs after the second verse/refrain.
Tools to write Bridge
We want the bridge to lead us back to (usually) the chorus by giving it more weight. Go introspective instead of telling more of the story. There are many ways to come up with ideas, but if you are stuck without ideas, think about asking questions of your chorus message.
Look at the main message of your chorus, probably your title. Question it. Come at it with a different perspective. Go deeper. See if any of the ideas that come out of your questions resonate with your message.
Chorus message=xx
What if xx wasn’t true?
What if xx didn’t happen?
How can I live without xx?
What can I do to change or fix xx?
Or have a wake up call: why does xx matter? Let go of xx.
Example:
Chorus message= it was love at first sight
What if it isn't true? Maybe it's just a physical attraction.
What if it didn’t happen? My life would be lonely.
How can I live without loving you? I'd be empty inside without you.
What can I do to change or fix this? I need better love glasses because I didn't see who you were on the inside.
Or have a wake up call: why does this matter? Let go of it. You're not in love with me so I need to let you go.
-------------
Depending on the angle of the verses, perhaps one of these ideas might be a good perspective for the bridge. It does bring up interesting idea I may not have thought of.