Overlay Settings on the left decides what the editor draws over the map; press + if it is collapsed. Use Defaults is fine for most of it, but a few of these show you a problem before it becomes one.
| Invalid Resource | Where a resource cannot be placed. |
|---|---|
| Invalid Depot | Where a Hatchery or Nexus cannot go. |
| Creep | Show creep, make it translucent, or hide it. |
| Pathfinder Regions | Draws the regions ground units path through, outlined in yellow. |
Pathfinder Regions shows exactly how ground units move: they hop between the yellow regions, and a connection is drawn green where one exists and red where it does not. If units are taking a strange route, look here first.
There is a limit on how many regions there can be. Past it the map does not load at all and you get "The map could not be loaded because it had too many obstructions". It comes from terrain chopped very fine or obstacles scattered everywhere, and this overlay is the only warning you get before it happens.