Extruding features is how you turn them from flat 2D shapes into 3D objects, providing them with height. For example, you can extrude power poles or trees so that you can see how tall they are.
Both of these are normally stored as point features. When you extrude them so that they have height, they become vertical lines. In the following screenshot, you can see an example of buildings and power poles that have been extruded to create 3D features:
From the previous screenshot, you can see that the buildings in this Scene started as a 2D polygon layer, which just shows the footprint of the buildings on the ground. By extruding them, you can now see their height above ground, gaining a greater appreciation of the spatial relationships between them and other features. The power poles were a 2D point layer that was extruded to show their height. Now, you can visualize the relationship between the buildings...