Traditionally, Advent wreaths are constructed of a circle of
evergreen branches into which four candles are inserted,
representing the four weeks of Advent. 

Ideally, three candles are purple and one is rose,
but white candles can also be used. 

The purple candles in particular symbolize the prayer, penance,
preparatory sacrifices and good works undertaken at this time.

The rose candle is lit on the third Sunday, Gaudete Sunday, when the priest
wears rose vestments at Mass on Gaudete Sunday because the faithful
have arrived at the midpoint of Advent, when their preparation
is now half over and they are close to Christmas.

The progressive lighting of the candles symbolizes the expectation and
hope surrounding our Lord’s first coming into the world and the
anticipation of his second coming to
judge the living and the dead.