Tri-band Orbi WiFi Systems have three separate radio bands: a 2.4 GHz band, a 5 GHz band, and an additional 5 GHz band backhaul. Your devices connect to the 2.4 GHz and 5 GHz bands while the router and the satellites use the 5 GHz backhaul to communicate with each other.
Quad-band Orbi WiFi Systems have four separate radio bands: a 2.4 GHz band, a 5 GHz band, a 6 GHz band, and an additional 5 GHz band backhaul. Your devices connect to the 2.4 GHz, 5 GHz, and 6 GHz bands while the router and the satellites use the 5 GHz backhaul to communicate with each other.
For more information, see Do I need to have a WiFi 6E router to use WiFi 6E capable products?