"blue-collar" generally means people who do physical, non-office work; and the jobs that they do. "white-collar" generally means office workers and management, and their jobs.

AFAIK it originates in the color shirts normally worn, i.e. if you're out working on engines you can't afford white shirt collars as they'll get stained.

The difference is a lot less clear in tech, where we're all "white-collar" in old-school terms but there is still a very wide scale of job-pleasantness and pay and everything else. I think when these terms originated, an entire productive industry of only white-collar folks was unimaginable.

("Productive" as opposed to, say, banking.)