This is a step for Life Cycle, that hides courses.
For each course, moodle saves the visibility for course (visibleold) and category seperately.
The final course visibility (visible) will be true, if both visibleold is true and the category is visible.
This is done, so that if you hide a course, then hide the category and then show the category again, the course remains hidden.
You can choose to display a course despite it's category being hidden, however, this is disregarded in case of a rollback.
When this step is executed, the original visibility (visibleold) of the course is saved.
Then the course will be set to hidden.
In case of a rollback, if the course is still hidden, the courses visibility (visibleold) will be restored to it's saved state
and the final visibility (visible) will be recalculated.
If a course is visible, this step is executed, you unhide the course, hide it again and then a rollback is performed, the visibility of the course will be restored to being shown.