Literally means: the green color (of vegetation) is so fresh that it looks like that juice will drip out. It's an adjective usually used for green mountains. e.g. 雨后的泰山苍翠欲滴
Imagine you are standing in a mountain covered with trees. It is in the spring, after a light shower new shoots grow out. The green vegetation looks so fresh that you can squeeze juice out of them. (or with some exaggeration, the juice will drip out by even without squeeze)