What do you mean by high impedance? I dont know what that means
You can just consider this an equivalent of disconnecting motor leads.
In the "brake mode", both leads are shorted (by being simultaneously connected to Vcc or GND), which causes the motor to slow down more rapidly and requires more torque to be moved once stopped - it operates exactly the same to a dynamo with an infinite load. In "freewheeling" high-impedance mode, leads are floating, and therefore, the load is removed - and the motor slow downs more gradually and is easier to operate.
[ Note that neither of these modes is really suitable for stopping the motor in place - to do this, you need to first enter brake mode for a very short time, and then reverse direction briefly. ]
All full H-bridge motor drivers support braking, but not all support freewheeling. This one does, which is nice (it potentially allows for more efficient PWM, for example).