Hams are permitted to transmit on "bands" of shortwave radio frequencies. Those "ham bands" are for the exclusive use of hams worldwide in much the same way as commercial radio and TV stations are assigned a frequency band in which to operate. Ham bands are wider than commercial frequency allocations and hams are allowed to move about within those bands -- unlike commercial radio snd TV stations. WSB radio in Atlanta, for example, operates on 750Khz and ONLY on 750Khz. Why do hams have multiple ham bands?
Shortwave radio signals travel by bouncing of earth's ionosphere. The ionosphere is more or less opaque to shortwave signals depending on the number of sunspots currently active on the face of the sun. Enough of the technotalk. Hams have multiple bands because, depending on the porosity of the ionosphere, shortwave radio signals bounce back to earth at different angles. Some ham bands are for short distance communication, others for longer distances. Got it?