You are entitled to get paid for every hour that you work.
An employer trying to rip you off might reduce your hourly rate down to the minimum legal hourly rate, and claim that they are paying you for each hour worked, but you don't actually have more money in your pocket. In that case you can and obviously would refuse to work more hours than the one in the contract. Say the contract says $20 per hour, up to 80 hours a month. Minimum wage is $10 per hour. You work 160 hours per month; they could try to claim that your wage dropped down to $10 per hour.
What this means in a normal case say with "80 hours per month" limit is that your manager or supervisor is not supposed to ask you to work over 80 hours a month, that you have the right to refuse, that on the over hand if you want to do overtime your manager can send you home. And if everyone agrees that you work 100 hours in a month your manager might have problems making the company's payroll pay up for 100 hours, or he might get in trouble for spending too much on wages, but you have the right to get paid.