"I Hear America Singing" by Walt Whitman is a celebration of Americans and the simple tasks or jobs they do every day. In this poem, "singing" is a metaphor for the sounds of each person's daily tasks. He calls these sounds "varied carols" and he lists many of the ordinary professions and the cheerful "singing" which is connected with each. Consider these lines:
The carpenter singing his, as he measures his plank or beam,
The mason singing his, as he makes ready for work, or leaves off work.
The poem as a whole draws attention to the simplest tasks, many of which are done without pay. A mother, a young wife, a young girl doing laundry or sewing, and a group of young men apparently just enjoying spending time together. This poem celebrates the everyday work of Americans in Whitman's day. Today, of course, there would be a different set of workers, perhaps, with a different set of "songs" (think technology and the sounds of many of today's jobs), and not all would be content with their tasks. The principal, though, is the same--drawing attention to the common man doing his job and enjoying it.
I have included a great eNotes link to Walt Whitman in case you need more information.