An underpaid person gets less money than they think is fair for their job.
When people talk about being underpaid, they usually mean that someone's salary or hourly wage isn't high enough considering how much work and responsibility they have. This can be a big problem if it's not just a little short, but significantly lower than what others in the same job are earning. For example, if you're working as a teacher and your friends who teach at other schools get paid twice as much for doing the same job, that would be underpaid. It doesn't mean they should be getting less money, it just means their pay isn't matching the value of what they bring to their job.
Getting too little financial compensation for one's work
Example: I am underpaid and overworked.
