This indicator will illuminate when the passenger side airbag is disabled. This can be because you set a bag or something in the seat that doesn't meet the minimum weight requirement to activate the airbag. I believe this is is around 50 lb with most manufacturers. Other times, when this indicator isn't present, the airbags are armed. A really handy feature with Tesla cars is the ability to open the owner's manual right from the infotainment screen in the center of the dash. You can click that to access the owner's manual and search for any of the terms you have questions about. Stuff like this is pretty easy to find if you take the time to look for it.
Assuming the seat sensor doesn't detect any weight thus the passenger airbag is disabled until it detects weight.
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