It means that mean voltage of generated impulses is proportional to the frequency of impulses. The mean voltage is calculated as surface of impulses generated in given time unit. The surface is bigger for bigger number of impulses. The communication lines going from neuron output to the neuron input are called axons. The axons are build from very small segments.
Impulses jump from one segment to the following one.
One segment transfers its content of ions to the other segment. This idea suggests that axons are in fact dynamic memories. It is true but memorizing of this type need deeper explanation that will be added somewhere to this site in the future. The impulses have left neuron outputs after the input voltage coming to the voltage has been converted to the impulse frequency. Therefore the brain must process decoding of impulses back to the voltage.