The bridge is symbolic of communication and union, whether it be between heaven and earth or two distinct realms.
For this reason it can be seen as the connection between God and Man.
It may be the passage to reality, or merely a symbol for travel and crossing.
In dreams, a bridge symbolizes the passage from one state to another, higher one, like an ascension; it is the end of one cycle and the beginning of another.
In many cultures it is the link between what can be perceived and what is beyond perception, or at least a change or desire for change.
It can be a test to see who is brave (who will cross).