What does it mean: "If part of a Rev Proc 2019-38 enterprise, select group # (see help)" ???

This question is referring to IRS Revenue Procedure 2019-38.

The "safe harbor" refers to the assumption that the rental property you own qualifies for to claim the deduction under section 199A with respect to a rental real estate enterprise as defined in section 3.02. 

Section 199A is the deduction for QBI (Qualified Business Income) which was enabled in the Tax Cut and Jobs Act of 2017. This is a deduction for operating a business that otherwise did not get a tax cut in the Tax Cut and Jobs Act of 2017.

You can read about QBI here (although since it was from 2018, the dollars amounts may be out-of-date).

I assume that the question about the group number of referring to whether or not you are going to treat multiple properties as one group (see the Rev Proc).