In classic Halachic literature, Shabbat begins at sundown.
Time pieces are imprecise.
As a result of this, the custom developed to light candles some specific amount of time before sundown. Most calendars in most cities in the US follow that format today. For traditional Jews, at that point candle lighting would be forbidden. Note that other cities may have different customs. In Jerusalem, the custom seems to be to light candles 30 minutes before sundown.