Schedule 2 is a new form this year. For 2018, the IRS completely
revised the 1040 form. They eliminated the 1040A and 1040EZ forms and
made the 1040 form a half page sized form that now has up to six
additional schedules that get attached to it (Schedules 1 - 6).
Therefore, anything that is reported on Schedules 1 to 6 used to be
reported somewhere on the 1040 form.
Schedule 2 is
where amounts due for AMT(Alternative Minimum Tax) or for repayment of the ACA Healthcare Advance Premium Tax Credit are reported. If you are required to make a repayment for an advance premium tax credit already received from a reduced health insurance premium amount purchased through the marketplace, then that premium credit repayment amount due is reported on Schedule 2.
Here is a sample of the Schedule 2 form:
Schedule 2