It seems as though Matlab displays intensity from black to white on a scale from 0 to 1, so I suppose your value would just be a decimal (or percentage if you multiply by 100%) from no light to total light. I am no expert, but your value would probably actually be brightness not luminance because luminance seems to have to do with area which doesn't really apply to computer pixels. Not positive though. You can read these two links for more information.
https://www.mathworks.com/help/matlab/creating_plots/image-types.html
https://en.wikipedia.org/wiki/Brightness
