绿林 literally means 'green forest', but as a historical term it refers to the Lülin band of outlaws from the late Western Han dynasty, and by extension any band of heroic outlaws or rebels in Chinese folklore and literature. The word carries romanticized connotations of Robin Hood-like figures who resist corrupt authority — you'll see it in 绿林好汉 ('greenwood heroes') and 绿林豪杰 ('greenwood champions'). It is not used for ordinary forests; for that, use 森林 or 树林.