In the Centaurus A picture the radio continuum shows radiation emitted by the synchrotron process, in which electrons interact with a strong magnetic field. Thus the radio continuum traces the magnetic field. Hydrogen gass emits radio waves at the particular wavelength of 21 cm, the image of the galaxy at the wavelength shows where the un-ionized hydrogen is. Most optical emission comes from starlight, so the optical shows where the stars are. The dark band across the middle of the optical image is a thick lane of obscuring dust. The most energetic photons, seen in the X-ray image, come from a jet emanating from the center of the galaxy.
"Continuum" radiation means radiation from a whole range of wavelengths.