Can someone explain simply what are ADU's? I was reading this post here: http://www.qsimaging.com/blog/understanding-gain-on-a-ccd-camera/

However, I still don't quite understand this statement:

"Gain on a CCD camera represents the conversion factor from electrons (e-) into digital counts, or Analog-Digital Units (ADUs). Gain is expressed as the number of electrons that get converted into a digital number, or electrons per ADU (e-/ADU)."

It seems ADU is a "digital number" but what is that? The units of ADU is "counts", but what are those counts? They are not electrons, it seems, then what are they? Time? I'm sorry if this is silly, but I just can't comprehend what ADU's physically are. It really stumped me from understanding the rest of the article.