Information that is digital is information represented by numbers (digits) or more broadly, information that can be measured in discreet, exact values. The opposite term is analog, which describes information represented along a continuous range, where there are an infinite number of possible values. Trite as it may be, the best way to understand the difference between digital and analog is to compare a digital clock to a traditional round clock with hands.
The display on a digital clock always shows one particular time, in numbers. A clock with hands, in contrast, is an analog device because the hands move along the entire circle of the clock face; at anyone instant the hands can be anywhere on the clock, displaying an infinite number of moments in time. Water is analog; ice cubes are digital. All common computers work only with numbers and are digital devices. If you want to use your computer to work with information from the natural world, which is almost entirely analog, you have to convert that analog information to digital form. See analog, A-to-D converter, and digitize.