Key here is the datasheet as linked in Jimmy Westberg's answer. The sensor will output:
The 4-20 mA vibration transmitters are piezo-electric accelerometers of compression type and provide a 4-20 mA output signal proportional to the true RMS value of vibration velocity.
So the output of this sensor is a current signal between 4 mA and 20 mA (not a voltage) that is proportional to the RMS value of vibration velocity. To read this sensors output the current will have to be converted to a voltage using a transimpedance amplifier (current-to-voltage converter) or measuring the voltage drop acros a well defined series resistor.
However as the sensor output is the true RMS value of vibrations in the specified frequency range (2..10,000 Hz) it is not possible to obtain the frequency (or to be more precise the wide frequency band) of the vibration with this sensor. To detect the frequency spectrum a measurement of the time waveform of the vibration amplitudes would be necessary.
This booklet about vibration measurement gives some more insight.
The RMS value is typically used in quantifying the vibration level:
The RMS value is the most relevant measure of amplitude because it both takes the time history of the wave into account and gives an amplitude value which is directly related to the energy content, and therefore the destructive abilities of the vibration.
The purpose of this sensor seems to be for monitoring of machinery where the actual time waveform of the vibration is of little interest. A single value (the RMS value) is sufficient to monitor the operation of the machine against a threshold value. It significantly simplifies measurement.
Experience has shown that the overall RMS value of vibration velocity measured over the range 10 to 1000 Hz gives the best indication of a vibration's severity. A probable explanation is that a given velocity level corresponds to a given energy level so that vibration at low and high frequencies are equally weighted from a vibration energy point of view. In practice many machines have a reasonably flat velocity spectrum.