You're expected to enter all of your income, sort out what portion of your income is taxable, determine the amount you can deduct from your taxable income, figure out what tax credits you're entitled to, calculate how much tax you already paid and what you still owe or should be refunded, and decide how you want to pay or be reimbursed.
In the simplest case it's straightforward.
But for anything other than the simplest case it quickly becomes a mess of additional forms (called schedules). Currently there's 3 main schedules (1, 2, 3) for Income Adjustment, Additional Taxes, and Additional Credits. And each of those references addition schedules and forms. Here's schedules 1, 2, 3: https://www.irs.gov/forms-pubs/about-form-1040 Here's a list of the other schedules: https://www.irs.gov/forms-pubs/schedules-for-form-1040 and a search for all the forms and instructions: https://www.irs.gov/forms-instructions-and-publications
The good news is, after all that, if you make a mistake the IRS will tell you what you actually owe and send you either a bill or a check for the difference.