Line 1a is actually related to Illinois Use Tax. You need to enter the value of merchandise you purchased to use in Illinois on which you did not pay the required Illinois Use Tax. This would include merchandise you purchased where you did not pay Illinois sales tax - sometimes this can apply to online purchases.
The Use Tax Smart Worksheet appears just above Step 8 on the IL-1040 when you are working in forms mode.
If this did not apply to you you should enter zero.
