Hi;
I will first say that while we have some idea of what coronal holes are, physically, there is still a lot of research to be done -- we especially do not know: why they form, what drives them to have outflows like they do, and why they seem to rotate at a different rate than other structures on the solar surface. Or, even how to identify them uniquely.
Coronal holes were so named because they look dark in X-rays, as places where the coronal emission decreases. Near the solar surface the magnetic field seems to be more uniform than we see in other places like sunspot groups,...They do show strong plasma outflows, hence their contribution to boosting the solar wind speed. Why they have these outflows is due to a balance of magnetic pressure and density. We do especially see a jump in the solar wind when the boundary of a coronal hole sweeps by the Earth.