In the most literal sense to digitize something means to turn it into digits or numbers. In a more practical sense it means turning something into the electronic language a computer can understand.
The opposite of digital is analog. A good example of an analog device is a wristwatch with hands, which move continuously around the face. Such a watch is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing a limited number of times (every tenth of a second, for example).
Humans experience an analog world. Vision is an analog experience, because we see infinitely smooth gradations of shapes and colors. However most analog events, can be simulated digitally. For instance, photographs in newspapers consist of an array of dots that are either black or white, a classic digital format. From a distance, the viewer does not see the dots, but only lines and shading, which appear to be continuous. Although digital representations are approximations of analog events, they are extremely useful because they are easy to store, transmit and manipulate electronically using a computer and a computer network.