Being "planar" means that a polygon must be flat. This is easy to do if you never use UPPITCH, DOWNPITCH, RIGHTROLL, or LEFTROLL between POLYSTART and POLYEND.
This is legal:
POLYSTART REPEAT 4 [FORWARD 100 RIGHT 90] ; a square is planar POLYEND
This is NOT legal:
POLYSTART REPEAT 3 [ FORWARD 100 RIGHT 90 ] FORWARD 50 UPPITCH 90 ; error: these instruction move FORWARD 50 ; out of the polygon's plane DOWNPITCH 90 ; FORWARD 50 ; DOWNPITCH 90 ; FORWARD 50 ; UPPITCH 90 ; FORWARD 50 RIGHT 90 POLYEND
If you want to create a polygon with a "bend" in it that leaves the plane, you can break up your polygon into simpler polygons that are coplanar.