Usually you are asked to verify your income by providing a copy of your paycheck stubs, a W-4 form or an income tax form (1040 or 1040 EZ). You will be asked about your savings, insurance policies, other investments and debts, including your monthly mortgage or rent payment, car and charge account payments.
This helps determine your general financial stability. You do not have to be rich to adopt; you just have to show that you can manage your finances responsibly and adequately.
Source: National Adoption Information Clearinghouse