A bridge in music, according to Wiki, performs the same function, in song or instrumental, as a bridge over, say, water does: it connects things. In song, it represents both a break from and a lead-in to the return of the original theme.
I was reminded of this song when I–ahem–cast scorn on a bridge from another in a comment attached to another post. “Making Plans”, written by Johnny Russell and a hit for my beloved Wilburn Brothers in the early 1960s, originally consisted of three verses. When the brothers decided to record it, Teddy took out the third verse and wrote a deceptively simple but effective bridge between the second verse and the final repetition of the main theme:
You’re making plans to forget me,
I’m making plans to miss you,
I’m gettin’ ready to grieve. . .
Johnny Russell agreed that the song was improved overall by the addition of the bridge. And best of all, Teddy Wilburn didn’t take a songwriter’s credit, although he certainly made a huge difference in the song’s effectiveness.