Neoclassical Theory: Your labor is worth exactly how much you get paid. Just like how a car is worth whatever it sells for, your labor is worth whatever you can get someone to pay you. This means that you are never underpaid, you are always paid the right amount.
Marxist Theory: Your labor is worth more than you are paid for it. Your employer hires you only if the amount of value you create is more than the amount he will be paying you. Otherwise there is no incentive for him to hire you. This means that all people are underpaid.