Answer:
In order to make an ethos, or ethical, appeal, convince the audience that you are a reliable, intelligent and can be trusted. Here is how you can achieve this: Provide personal experience or know someone whose experience can relate to what you are talking about. Use detailed and recent research in your argument.
There are several things that might be symbolized by the word bridge in this line from the poem, but perhaps one thing would be the connections that you form in life.