Navigation stack helps you show the Subschema or PCR’s accessed and the first entry is the object you last accessed.Let me illustrate this with the snapshotspretty handy one,huh!
PCR X015 using TCD PE02
from X015 navigating to X016
now when we click Stack(Shift+F4) or Goto ->Stack it will show in a new pop up window X015 which was first accessed
and when click on X015 and try the same it will result in showing the Navigation Stack is empty as anticipated