A coronal hole is an opening in the sun's magnetic field through which gas can easily escape into space.

The holes are relatively cool in temperature as compared to the active regions nearby -- such as the bright region on the lower left portion of the solar disk -- the cooler temperature is one of the reasons for the darker appearance. Hinode, a Japanese mission in partnership with NASA, NAOJ, STFC, ESA, and NSC, currently in Earth orbit, is studying the sun to improve our understanding of the mechanisms that power the solar atmosphere and drive solar eruptions.

Source: HiNode homepage