We define "retail traders activity" as share of volume traded by retail traders at a given ticker at a date vs overall volume traded by all retail traders across all tickers at a date. Basically, that kind of a metrics tells you how strong retail traders are preferring "trading a given ticker" vs "trading any other tickers". "Any other tickers" in this context means "all US stocks and ETFs".
Retail Traders Activity Percentage on chart
You can enable this line on your charts using the Other Data menu in the top toolbar. Open it and then enable Retail Traders Activity Percentage.
In example, the picture below tells you that at Jan 26th 2021, 9.318% of all retail trading activity of this date was focused on GME.
Retail Traders Activity lists
You can also have market-wide ratings of tickers traded by retail traders at a date. Read more here.