When working with Height data and Imagery, it is sometimes useful to view it in 3D. QGIS makes this straightforward, and we'll run through it step by step here.
Organise your layers - ensure you have a DEM in your Layer tree, alongside anything else you'd like to show in the output.
Open the 3D view in the View Panel --> New 3D Map View
New 3D Map View in the Main Menu
This will open up a new Map view:
3D Map View and the normal 2d Map view beneath it
As you can see, the 3D view isn't currently in 3D! This is because we have to tell the view which layer to use for Elevation information. You can do this by selecting the Wrench tool to open the Configuration Panel:
Click Terrain
Type = DEM (Raster Layer)
Elevation = pick your DEM layer
Vertical Scale = default is 1, but you can exaggerate the height by increasing this number
Click OK
Map View Configuration
You should now have a 3D-enabled Map View!
Use your mouse to navigate - click and hold the scroll button and move your mouse up and down to lower the view angle, or you can hold shift and click and drag.