It means that the computers have gone through their power-up diagnostic self-testing, see no problem and the car is ready to go !
Among other uses, it warns Emergency Responders that the engine may start at any time. If your car is not READY then the engine is REALLY off. https://techinfo.toyota.com/techInfoPortal/staticcontent/en/techinfo/html/prelogin/docs/2ndprius.pdf Page 14 of the PDF, page 11 of the manual It would be bad to endanger the Emergency Responders.
I thought it was a mood indicator for your date. No wonder I've been slapped so many times since I bought the Prius. Tom
For best results, let it warm up a bit and then go easy on the gas. Often it works better if you use a light touch and feather it bit, although some prefer battery operation. Tom