Absolute magnitude is the measure of the intrinsic brightness of a celestial object. It's equal to the apparent magnitude an object would have if located at a hypothetical distance of 10 parsecs (32.6 light-years). This way the actual brightness of stars and celestial objects can be compared despite being located at various distances from Earth.
The absolute magnitude can be expressed for different wavelengths. Usually the visual band of the electromagnetic spectrum is used and this is known as the "absolute visual magnitude". The "absolute bolometric magnitude" corresponds to the total luminosity radiated across all wavelengths of the electromagnetic spectrum.