GIS Lab (raster algebra)

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).