Literally: cage – bird – (jail) cell/pen – monkey
Alternately: Unfree; specifically, having had one’s freedom taken away. Living like a bird – or monkey – in a cage, unable to do what you want.
Notes: This phrase comes from the poetry of Bai Juyi (Japanese 白居易 = Haku Kyoi), a Tang-era official and, well, poet. Apparently Bai and his friend Yuan Zhen were both demoted and exiled, and perhaps placed under other restrictions, and Bai came up with this metaphor to describe how he felt about it.