Question

They used to tell me I was building a dream, And so I followed the mob— When there was earth to plow, or guns to bear I was always there—right on the job. Which best restates the main idea of these lines?

Answer 1

The main ideas is that whoever is narrating these lines, believes that working hard will help get them to their dream. 

Answer 2

These lines belong to the song "Brother, Can You Spare a Dime?". This song is about the Great Depression that hit the United States in the 1930s. The main idea of the excerpt can be restated by explaining that the men that were waiting for food used to fight for the American Dream.