What is close marching animation?

When you send your legions out you will see a line indicating the route and an animation of the troops moving. You will also see this for any other legion from other players marching.

The 'close marching animation' will hide the troops marching. This is mainly useful for events where many players are close together and sending troops at the same time, eg during Reign of Chaos events. When there are lots of troops shown on the screen it can be hard to select areas on the map so this prevents the troops getting in the way.