The All list is used to collect all tasks assigned to any list into one view. The All list is a built-in list and cannot be renamed or removed and tasks cannot be assigned to it. It is always shown at the top of the lists in the Lists view.

The All list can be filtered to only shows tasks from specific lists. In Settings -> General there is a section called "List Filters". In that section is the setting "All" List Filter. By enabling the filter and un-checking lists, the un-checked lists will be filtered from view in the All list.

In this case we have a list named Someday that is full of all of the tasks to be completed someday. These are not tasks to take action on so we've selected to filter out the Someday task list. Below you can see that the All list is now filtered and the number of tasks in the lists has been reduced by the number of tasks in the Someday list.