I’ll go for the direct answer
“Festoon” is the shape of the bulb. It’s basically a rectangular bulb/LED with a connector on each end.
“CANBUS Error Free” means that it has a little resistor in it so that it doesn’t trip your smart’s rapid turn signal flash.
So, if you’re replacing the license plate lights, you’re going to want a festoon CANBUS LED.
I recommend getting CANBUS in every LED. In my experience, even if the smart detects too little resistance in the license plate lights it’ll trigger the rapid flash.