What does "Set remaining work to zero" mean in a column?

If you check this option, the remaining work in a backlog item or task will be set to zero when the backlog item or task is moved to the column.

For example, if you have a column that represents test or approval activities, and no more of the estimated work remains at that point, it's useful to select this option for the column:

The option is available only for columns with status "In progress" and "To test".