These tiles are used to display the pathing map. If you first press Alt + Shift + A while in the editor menu, and then press Alt + D while actually editing a map's landscape, you will see the tiles you are asking about.
The first Alt + D press will show you the white tiles with the red borders. Red borders indicate that units cannot cross them. There are eight possible positions for the red borders, one for each side and one for each corner.
Pressing Alt + D again will show you the numbered tiles. All tiles within a group of connected tiles will have the same number. So if a tile is numbered 1, it cannot be reached from tiles numbered 2, 3, 4, and so on, and vice-versa. The exception to this rule is gatehouses, they create new pathing groups even though they might provide access between two groups of tiles.
Pressing Alt + D one more time will return the landscape to normal.
-DOOMBRINGER-
"DOOMBRINGER, make a parking space IMMEDIATELY!"
"DOOMBRINGER declares it to be SO!"
"DOOMBRINGER, I command you to lay siege to THE ISS!"