They could refer to Green Dragon of the East, or 东方青龙 in Chinese, and White Tiger of the West, or 西方白虎 in Chinese, two of the Four Cardinal Points of directions in Chinese culture. The other two are Vermillion Bird of the South, or 南方朱雀 in Chinese, and Tortoise and Snake of the North, or 北方玄武 in Chinese.
Here is a picture about these Four Divine Symbols or Four Guardians. In this picture, there is another symbol in the middle, which is Qilin, or Kirin, or 麒麟 in Chinese.
Please notice that the directions in this context are different from what we usually associate with, since in the Book of Rites, which mentioned that in the front is the Vermillion Bird, whereas in the rear is the Tortoise and Snake. On the left is the Green Dragon, on the Right is the White Tiger, or 前朱鸟而后玄武, 左青龙而右白虎 in Chinese.
Also these four points could refer to four seasons, i.e, Green Dragon of the East refers to the Spring, Vermillion Bird of the South refers to the Summer, White Tiger of the West refers to the Autumn, Tortoise and Snake of the North refers to the Winter.
Of course, these four points could refer to four different elements in Five Element Theory. Green Dragon of the East points to the Wood Element, whereas the White Tiger of the West points to the Metal Element. The Vermillion Bird of the South points to the Fire Element, whereas Tortoise and Snake of the North points to the Water Element.
They could refer to four different apparatuses in the body. Green Dragon of the East points to the liver, whereas the White Tiger of the West points to the lung. The Vermillion Bird of the South points to the heart, whereas Tortoise and Snake of the North points to the kidney.
The reason behind that why the dragon represents Yang, whereas the tiger represents Yin could be the hexagrams they stand for in the Book of Changes, i.e., ䷲ or Thunder, or 震 in Chinese, and ䷹ or Swamp, or 兑 in Chinese. Because ䷲ or Thunder represents the first son, where as ䷹ or Swamp represents the third daughter, as a result, Green Dragon of the East is Yang, whereas White Tiger of the West is Yin. The following picture could illustrate this point.
