"Adjustments to income" are the changes you make to your "total income" to get your "adjusted gross income". These are things that come between the "total income" line and the "adjusted gross income" line on form 1040 (i.e., lines 23 through 36 on the 2014 form). These are sometimes called "above-the-line deductions"; they are deductions that you can use even if you don't itemize deductions. You can see an IRS page about them here.
An IRA contribution is indeed an adjustment to income. On this IRS page explaining which form to use, you can see that it says you can use 1040EZ if "you do not claim any adjustments to income", but you can use 1040A if "your only adjustments to income are the IRA deduction, the student loan interest deduction, the educator expenses deduction, the tuition and fees deduction". That makes it clear that you can't use 1040EZ if you want to claim a deduction for IRA contribution.