Mtreeview Extension

Hi All,

  For Treeview structure, I searched many extensions and I found "mtreeview" extension best for me. I have downloaded its demo and it is working nice. Now I want to implement same demo in my project but I am confused in 2 fields which are used in that. What is the use of "lft" and  "rgt" field in that.

I found only this much information about these fields :

lft : the field to be considered as the ‘left’ in nested set.

rgt : the field to be considered as the ‘right’ in nested set.

Can anyone explain me in details about these 2 fields??

Thanks!!!