Routing is how sims get from place to place. You can make areas non-routable by painting them in the paint/sculpt terrain. You can paint terrain "sims non-routable", so sims can't reach that area, or "camera non-routable", which prevents the camera from crossing that terrain. In addition, underwater areas and areas that are steep are automatically non-routable. Make sure there are no tiny patches of routable terrain inside non-routable terrain, or sims will get stuck there, lagging your game until they die. When you are finished making terrain, and placing lots and roads, you should "rebuild routing data", available from the top toolbar in Create A World.