Raster algebra is one of the essential things you will need when working with RS and GIS datasets. One of the most common examples would be to replace the no-data value e.g. from -9999 to -32768 or whatever is required for your purpose and the software you are using.
For a summary and examples check e.g.: Raster algebra in QGIS with RasterCalc
In differnece to the GIS-Lab webstite statement it is not anymore necessary to install anything in a complicaed way. The RasterCalc tool they are describing can be found in the main repository via the standard plugin search and install (plugins -> fetch plugins).