Question: What do the individual candles of the Advent wreath symbolize?
Each candle represents one week of advent. A tradition says that each week of advent represents 1000 years, or four weeks of advent represent 4000 years from Adam and Eve to the birth of Jesus.
There are four candles: three are purple and one is rose. The purple candles symbolize prayer, penance and sacrifice. The rose candle lit on the third Sunday of advent (Gaudete Sunday). It represents rejoicing that our wait for the birth of Jesus is nearly over.
There is also deeper meaning for each candle:
The first candle, the "Prophet's Candle" reminds us that Jesus is coming.
The second candle, the "Bethlehem Candle" reminds of of Mary and Joseph's journey to Bethlehem.
The third candle, the "Shepherd's Candle" reminds us of the Joy the world experinced at the coming birth of Jesus.
The fourth candle, the "Angel's Candle" reminds us of the angel's message of Peace and Goodwill.
Bonus question: What do the evergreen branches of the Advent weath symbolize?
The evergreen branches signify continuous life. They are placed in a circle, without beginning or end symbolizing the eternity of God. So the evergreen wreaths symbolize the immortality of our soul and the everlasting life promised to us though Christ.
https://www.catholiccompany.com/blog/the-advent-wreath-tradition-meaning
http://www.catholiceducation.org/en/culture/catholic-contributions/the-history-of-the-advent-wreath.html