You hook up the output leads to a voltage datalogger or an oscilloscope, and then expose the meter to a source of light. The voltage signal you measure is proportional to the intensity of light at the spot where you have placed the meter. W/m2 is watts per square meter, or power of light received per square meter of exposed area.
An example is that on the surface of the earth the maximum intensity of solar radiation is about 1000 W/m2. Your meter would read 1000 x 4.55 or 4500V. That seems rather high to me. Check the label again to see if it doesn't say 4.55 mV/W m2.