Groundwave is the propagation of radio waves close to the surface of the Earth.
Lower frequencies, especially AM broadcasts in the mediumwave and longwave bands (and other types of radio even below that), travel most efficiently this way. This is because they are more efficiently diffracted by the figure of the Earth due to their low frequencies.
Ionospheric reflection is taken into consideration as well -- the ionosphere reflects frequencies in a certain band, which often changes due to solar conditions. Because the solar wind "blows" the ionosphere toward the Earth on the day side, and away from it on the night side, this natural radio "mirror" is much closer to the surface during the day -- preventing skywave propagation from being very effective in daylight hours.