It's where you take a gallon of water, put 4-5 lipton (or equivalent) tea bags in there and let it sit out in the sun for 1-2 days. It steeps the water in the tea and gives it a strong and unique flavor.
http://www.ljcfyi.com/blog/2007/july2007/sunTea.jpg
http://www.ljcfyi.com/blog/2007/july2007/sunTea.jpg