If a job is unpaid, that means you do it without receiving any money in return. Your unpaid internship at the town newspaper will give you lots of experience, but no income.
An unpaid worker hasn't received their paycheck yet, and an unpaid bill means you haven't sent the money you owe to the electric company or dog walker. This adjective is also useful for describing volunteer work, like an unpaid position at the local animal shelter. Unpaid was first coined in the 14th century, from un-, "not," and pay, which has a Latin root meaning "satisfy a creditor," or literally "make peace."