By default, "Simultaneous tasks" setting is disabled. When user tap a task on "Today" tab, if there is another task running, that task will be stopped automatically.
With "Simultaneous tasks" option enabled, User can keep multiple tasks running at the same time. When user starts a task on "Today" tab, it will not stop current running task.
This option is useful if user frequently tracking multiple tasks at the same time. E.g. reading book while commuting to work and user want to track time spending on both of them.