I think Ham radio means hobby and amateur radio, I.e. not professional. Radio is a type of radiation at very long wavelength. From the wiki:

long waves can diffractaround obstacles like mountains and follow the contour of the Earth (ground waves), shorter waves can reflect off the ionosphere and return to Earth beyond the horizon (skywaves), while much shorter wavelengths bend or diffract very little and travel on a line of sight, so their propagation distances are limited to the visual horizon.

Others have explained mesh pretty well.