That means several "subexposures" (images) are taken and stacked on top of each other with special stacking software, which aligns each "sub" and combines them to create a final image with less noise and more signal than a sub. Effectively this increases exposure time without the need for an impossible single super long exposure.
A "Lumanence" image, typically taken with a Clear, Lumanence or Ha filter is used in Photoshop as a seperate layer for processing detail and sharpness and then blended with an RGB image to create the full colour final image called an LRGB , to reflect the processing used.
Detail and sharpness processing is done on the Lumanence layer, because it usually has more signal and less noise than RGB subs, and can stand more processing. It also allows the use of shorter RGB exposure times, since there is less need for detail in them.
Someone will trash this explanation, Im sure others can do it better
