The dimmer control provides all levels of light between dark and bright. A digital device moves values that fall between two numbers to either the higher or lower number. If the light control were a preset knob with only four levels, the control would provide only four discrete levels of light. This is an example of a digital device. Some signals are not ready to be digitized by a DAQ board when they come out of a sensor.
Signal conditioning prepares signals to be used by the digitizer.
A signal can be corrupted by a noisy environment - not in terms of audible sound but in terms of electronic noise. Signal conditioning helps shield signals from electrical noise. In the microphone example, the signal was amplified to make the signal bigger. Another type of signal conditioning is attenuation, which makes the signal smaller. Signal conditioning also can act as a circuit breaker, isolating the computer from dangerous signals.