Hi Babek,
The output of the Get Raster Properties tool is a result object. If you are running the tool from the Toolbox, that means the results will be available in the Results window. Here are the instructions on opening and using the Results window:
Using the Results window—Help | ArcGIS Desktop
If you're using ArcGIS Pro, the results will be written to the Geoprocessing History. Here are the instructions for using that:
http://pro.arcgis.com/en/pro-app/help/analysis/geoprocessing/basics/run-geoprocessing-tools.htm#ESRI...
If you're just looking for the Raster Properties, you can also access them without the Get Raster Properties tool by right-clicking the raster dataset in the Catalog window and selecting "Properties." The statistics you're looking for will be listed there under "Statistics."
Raster dataset properties—Help | ArcGIS Desktop
I hope that helps!
-Lauren