To digitize something means to convert it from analog to digital. For example, an analog audio signal received by a microphone is digitized when it is recorded by a computer. Computers must digitize analog input because they are digital devices and cannot process analog information.
Digital recordings are created by taking samples of analog data, typically at the rate of several thousand per second. For example, CD audio is digitized using a sample rate of 44.1 kilohertz (kHz). That means the analog signal is sampled at 44,100 times every second. The greater the sampling rate, the better the quality (or higher the fidelity) of the digitized file.
Since digitizing takes small samples an analog signal, the result is an estimation of the original data. Therefore analog data is actually more accurate than digitized data. However, as long as a high enough sampling rate is used, humans perceive digitized audio and video as a steady stream of analog information. Because computers can edit digital data and copy digital files with no loss of quality, most of today's media is created and saved in a digital format.