"Coronal holes are areas where the Sun's corona is colder, hence darker, and has lower-density plasma than average because there is lower energy and gas levels. Coronal holes are part of the Sun's corona and are constantly changing and reshaping because the corona is not uniform.[1] The Sun contains magnetic fields that arch away from areas in the corona that are very thin due to the lower levels of energy and gas,[1] which cause coronal holes to appear when they do not fall back. Thus, solar particles escape at a rate great enough to create a lower density and lower temperature in that area.[2]"
Coronal Hole Magnetic Field Lines-image shows what happens when the field lines do not fall back, and instead allow solar wind to escape