"Rob Thomas doesn't sing, he just yarls"
Although it has been said that Eddie Vedder's yarling was taken from people such as Ray Charles and Bruce Springsteen, the similarity between Vedder, Charles, and Springsteen is nowhere near as close as Vedder, Stapp, Wade, and Band.
Essentially, it has the same basic meaning as 'eh'; 'meh'.
The actual defenition is usually deemed as whatever the hell the author (speaker) feels compelled to make it mean.
Person B: "Yarl, I got shit to do.."