Analog is the opposite of digital, and I can only explain analog in relation to digital. Analog refers to things that are in a continuous flow or that have an infinite number of values-things that are "analogous" to real life.
For instance, a watch with hands is an analog device, because as the hands sweep over the watch face, time can be shown in a continuous flow, with an infinite number of possible increments. A digital watch, on the other hand, displays time in finite chunks-seconds or tenths of seconds or perhaps even smaller chunks, but at anyone moment there's one specific number showing, always countable and limited.
A photograph represents a scene or a face in analog form, in continuously varying tones of grays or colors. When you scan a photograph to get it onto your computer screen, the scanning process breaks up those continuous tones into digitized (digital) bits of information, because your computer can only understand digital things.
Guy defines analog as "anything you can read in direct sunlight." It is possible to make an analog computer, but digital computers are much easier to build, faster, and more reliable.