When the design navigation mode is on, as you move the cursor through model space, each component the cursor moves over highlights (in model space as well as in the mechanical browser) and a tooltip shows the hierarchical structure of the component. If you press the CTRL key, the tooltips become transparent.
In the System Editor, in the Acad/M > CIT branch, the key NavigationToolTipOpacity controls the opacity of the tooltip. This key contains the percentage of opacity; 100 means that the tooltip is opaque; 0 means that the tooltip is invisible.
The key NavigationMode shows the current state of the Design Navigation Mode. If NavigationMode is 0, it means that the Design Navigation Mode is off. If NavigationMode is 1, the Design Navigation Mode is on.