Slope is one of the most basic parameters that can be derived from a DEM. It corresponds to the first derivative of the DEM, and it represents the rate of change of the elevation. It is computed by analyzing the elevation of each cell and comparing this with the elevation of the surrounding ones. This recipe shows you how to compute slope in QGIS.
In the Processing Toolbox option, find the Slope algorithm and double-click on it to open it:
Select the DEM in the Input layer field.
Click on Run to run the algorithm.
Slope is calculated from a DEM elevation model by analyzing the cells around a given one. This analysis is performed by the slope algorithm from the GDAL library.