Dos Passos (with these novels in particular) sometimes has a reputation for being difficult. It's certainly true that his style of jumbling long stretches of character biography alongside shorter pieces and sections of news headlines, song-lyrics, etcetera, can be disorienting at first, but once you get used to the scrapbook effect it produces, it makes a lot of sense.
Most of the book is taken up with the narratives of individual characters' lives. It's hard to describe a plot, as the book follows one character for maybe a few dozen pages, maybe hundreds, before switching to another character, perhaps to return later on (some characters recur, some appear once and then disappear again). What links all of the characters is that they are Americans living their lives in the early 20th Century. beyond that there's a huge variety: some are tremendously successful, some lead hard, directionless, arguably meaningless lives. What Dos Passos does is to accord all of them the respect of his narrative. He follows bums and itinerant workers with the same eye he follows businessmen and artists. The end result is a kaleidoscopic vision of early 20th Century America.
If you're a bit worried about the commitment (this is a long, long book), I'd encourage you to try reading the novels separately and see how you go. They were originally published as three medium-length stand alone novels, and taking a breather between installments certainly helped me get through. In the end, it's well worth it to experience an incredible period through Dos Passos's impressionistic, shifting, but always lucid stories.