Go to the menu Settings, then choose Board settings on the left.
This feature display a shadow around the task when a task is moved recently.
Set the value 0 to disable this feature, 2 days by default (172800 seconds).
Everything moved since 2 days will have shadow around the task.
When you share a board, the page will refresh automatically every 60 seconds by default.
When your web browser is open on a board, Kanboard check every 10 seconds if something have been changed by someone else.
Technically this process is done by Ajax polling.