Coronal holes are areas in the Sun's atmosphere that appear dark in X-ray and ultraviolet images of the Sun. The plasma (electrified gas) in these regions of the Sun's corona (it's upper atmosphere) is cooler and more dense than in other parts of the corona.
In most places on the Sun, magnetic field lines loop back onto the Sun's surface. Plasma in those regions flows along the field lines, giving rise to features like coronal loops and solar prominences. However, the magnetic field lines in coronal holes do not loop back down. Scientists call these "open field lines." Plasma flowing upward through the corona can more easily escape into space along these open magnetic field lines at coronal holes. This escaping plasma produces faster flowing solar wind than at other locations on the Sun. The fast solar wind races outward from the Sun at speeds of 800 kilometers per second (500 miles per second), about twice as fast as the slow solar wind that flows away from other places on the Sun.
At the peak of the sunspot cycle (called solar max) when the Sun's magnetic field is very active and disturbed, coronal holes can appear almost anywhere on the Sun. At solar min, when sunspots are fewer and the Sun's magnetic field is more stable, coronal holes usually appear near the Sun's North and South Poles. Coronal holes are most easily seen in ultraviolet and X-ray images of the Sun.