Custom hire is contract hire OR machine hired work. It's a payment for work or services that have been done on your farm to another self employed person or business.
From IRS Instructions for schedule F, page F-6;
Line 13
Enter amounts paid for custom hire or machine work (the machine operator furnished the equipment). Don't include amounts paid for rental or lease of equipment you operated yourself. Instead, report those amounts on line 24a.
https://www.irs.gov/pub/irs-pdf/i1040sf.pdf
