An income-sharing agreement (ISA) is a contractual agreement between a student and his school. Students agree to accept money borrowed from the university to fund their education. In exchange, they agreed to pay a certain percentage of salary to the university after graduation (in the next few years).
The amount you repay (think of the minimum repayment amount) will increase as your income increases. So basically, as you progress in the professional field and start to increase your salary, the income sharing agreement will come into effect and take up more and more of your income.