what is the qbi at risk loss on Schedule F?

QBI is Qualified Business Income required to compute the Qualified Business Income Deduction.

IRS Publication 925, page 4 discusses at-risk carryovers here:

Carryover of Disallowed Deductions 

In the case of an activity with respect to which any deductions or credits are disallowed for a tax year (the loss activity), the disallowed deductions are allocated among your activities for the next tax year in a manner that reasonably reflects the extent to which each activity continues the loss activity. The disallowed deductions or credits allocated to an activity under the preceding sentence are treated as deductions or credits from the activity for the next tax year. For more information, see Regulations section 1.469-1(f)(4).