State of charge (SoC) is the battery's current energy level as a percentage of its usable capacity. It is displayed on the dashboard and in charging apps, and it affects how quickly the battery can charge.
DC fast charging is quickest at low and middle SoC and slows down as the battery approaches full. Many manufacturers suggest a routine daily charging limit of around 80-90% and charging to 100% before long trips, though guidance varies by battery chemistry.