What does high impedance value mean?
In this context, it means that the digital signal line is not being actively driven to either a 0 or a 1. When a signal is in a high-impedance state, another driver (i.e. another gate output) can control the logic level of the signal.

This is a common way to allow multiple drivers to share the same data line. There is some control logic that determines which driver is allowed to put data on the line, and all other drivers must be in the high-impedance state.

If you are writing behavioral Verilog, you can assign the value Z to a signal to put it in the high-impedance state. Likewise, a simulator should show the value Z for a high-impedance signal.