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

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(NASDAQ: HOOD) Delayed: 112.57 -0.76 (0.67%)
Previous Close $113.33    52 Week High
Open $113.35    52 Week Low
Day High $116.40    P/E N/A 
Day Low $111.02    EPS
Volume 19,717,558