Issues accessing galleries after editing easy2_dirs table

Hi,

After manual edition of the easy2_dir table, I encounter a problem with the path to the root folder.

The web site use three folders (which are towns).
Easy2Gallery does not list the folders anymore. As the path, it chains the names of the miscellaneous galleries, like if they were subolders of each other, whichever the selected gallery.

I compared the content of the table "easy2_dirs" with thus of another site running well and I cannot understand what I'm doing wrong since the "parent_id", "cat_id" and "cat_level" fields seem being correctly set.

I assume that I'm missing something about the role of the "cat_left" and "cat_right" columns and how they should be set. Please see the two attached screenshots.

Who can help?
Thanks a lot.

Deleting and creating again the "easy2_dirs" table solved the problem. So, I see that the "cat_left" and "cat_right" records have their importance.
They seem being auto-incremented by the module. (Furthermore, cat_left is a KEY.)

But what is their role?
What do those values represent?