This is the second Sunday of Advent. The Advent wreath may have purple or blue candles in it. Whichever color there is, this is the Sunday to light the second one. Actually, the first one is relit, as well as the second one for the first time. The second candle is a symbol of Christ the Way.
The Advent wreath at my church.
The white candle in the middle is called the Christ Candle, and it is lit on Christmas Eve or Christmas Day.