Ghost mode changes the transparency values of bodies and highlights the edges of them. This way you are able to see what's "inside" or "behind" certain bodies.
1. Let's say, you have a complex geometry and you need to understand if there are hidden edges
2. Tap the orientation cube in the top right corner to open the menu.
3. On the top bar select Appearance.
4. Show hidden edges by switching the radio button to active. Now you can see the edges of bodies and what's inside / behind them.