Plain-language answer: A skip zone is the region between the practical end of ground-wave coverage and the first useful returning skywave for a frequency and antenna geometry.
Why operators care
Stations inside it may be weak while both closer and farther stations are readable. Frequency and launch angle move its boundaries.
What to compare on air
Map reports by distance and try a lower band or higher-angle antenna pattern for regional coverage.
The common mistake
Increasing power to solve a geometric skip-zone problem.
Forecast boundary
Space-weather indices and propagation models describe probabilities and regional conditions. A usable contact still depends on the exact path, time, frequency, antenna pattern, noise, power, and current observations.
