The Layer viewer is used for displaying a non-transformed, pre-effects version of your currently selected layer. This is used for accurate masking and during 2D tracking.
Accessing the Layer panel
The Layer panel is always linked to the Viewer. You can display the Viewer or the Layer panel but you can't see them both at the same time.
To use the Layer panel, simply switch to it using the tab next to the Viewer, then select a layer on the timeline.
You can switch between the Viewer and the Layer panel at any time when in a composite shot timeline.
The Layer panel shows your selected layer before any transformation or effects are applied. This can be very useful when compositing: even if you've animated your layer so that it is difficult to work on directly in the Viewer, you can switch to the Layer panel to get an undistorted view.
Check out the Masking section for more details.
The Layer panel is used for positioning 2D tracking points. For details on HitFilm's 2D tracking head over to the Tracking section.