During the late 19th century, the main grievances of labor generally revolved around wages and working conditions. In this era, the US economy was industrializing rapidly. It was a time in which most workers were doing jobs that did not require much skill. Because of this, the individual workers were not valued very highly by their bosses. The firms would pay them low wages and they had to work in poor conditions. If the workers complained too much about these things, they could easily be replaced because they were not highly skilled and because there were plenty of other people looking for jobs.
Because of this, labor unions formed. They were mainly concerned with trying to win higher wages and better working conditions since those were the major grievances on the part of labor.