Songwriters often use the word ‘bridge’ to describe a part of the song. Roughly speaking, a bridge is the only part of the song (apart from the solo), which doesn’t repeat itself. The purpose of the bridge is to build the tension leading up to the climax of the song or to lead a song to its conclusion.
A song usually follows the verse-chorus/verse-chorus form that can make the listener loose interest in the song pretty quickly. Hence the songwriter inducts a bridge to break away from the monotony of the set pattern and inject some excitement into the song.
When it comes to writing pop/rock songs, songwriters usually place the bridge right before the instrumental solo/break. The bridge is a pathway that leads the song to a new level with the aim of bringing it smoothly back to the chorus. Some examples:
Mr.Big – "To Be with You": where Eric Martin says, “why be alone/when we can be together baby..” right before the solo.
Bon Jovi – "Always": Ritchie Sambora breaks into the solo and Jon Bon Jovi sings, “If you told me to cry for you.."
Police – "Every Breath you Take" The bridge kicks in with a guitar chord strum and Sting starts singing “Since you’ve gone, I've been lost without a trace..”.
At times the part that separates a verse from the chorus is also considered as a bridge, however, more often than not it’s referred to as the ‘pre-chorus’. Lyrically and rhythmically a conventional bridge should bring the song closer to its climax while the pre-chorus simply connects the verse to the chorus. When it comes to moving a song further nothing plays a bigger part than the bridge. Think of it as connecting the story that has been told in previous verses and choruses to its conclusion.
Rhythmically and melodically the song changes as well. The instruments subside to give the vocals a lot more space before the guitar breaks into a full-fledged finger picked solo. A songwriter needs to listen to a lot of songs from different genres to get a feel for the bridge.