That's the work that the system is doing to the environment. I'm doing work to the environment. The work is negative, which means that work is being done to the environment.

功的大小,就是矩形的面积,这样我们就实现了一台热机，冷却，加热，加热,冷却，系统回到了初态。

麻省理工公开课 - 热力学与动力学课程节选