Explainer-Robinhood makes most of its money from PFOF. What is it?

Go back to Explainer-Robinhood makes most of its money from PFOF. What is it?
(NASDAQ: HOOD) Delayed: 108.13 +13.03 (13.70%)
Previous Close $95.10    52 Week High
Open $101.10    52 Week Low
Day High $109.71    P/E N/A 
Day Low $98.77    EPS
Volume 50,408,743