To multiplex the digits is to illuminate one digit at a time. The rotation is done rapidly. Persistence of vision causes our eyes to perceive the entire display as though all digits are lit simultaneously.
This provides benefits:
* only one driver IC is needed (rather than several)
* reduced current need (because only one digit is ever lit, rather then all digits simultaneously).
Seven data wires come from the driver IC. These fan out directly to all the digits.
However each digit's path to ground is also turned on and off by an individual control wire, coming from the driver IC.