If you're uncompensated, you haven't been paid money you're owed. Workers who are uncompensated for the overtime hours they've worked may decide to go on strike.
You can use this adjective to describe volunteer jobs, like an uncompensated internship that you do for school credit rather than a salary. It's also useful for situations in which you should be paid but haven't been. If you're injured at work but get no financial assistance, you're uncompensated. And if you return a pair of shoes by mail but don't get a refund, you're also uncompensated.