In general a file manager is a program that gives some kind of interface to the file system and that show in a graphical or textual way the files and directory, usually a file manager allow to do some standard operations such as delete, rename copy/paste and other typical operations that you can do on files. Twin-panel file managers have obligatory connected panels where action in one panel results in a reaction in the second.
So for example you could move a file from the first to the second panel, or copy it. In this roundup I’ll show you some of the most used Twin-panel file manager available on Linux, sometimes they are also called Orthodox file managers or command-based file managers and in general they have three windows (two panels and one command line window).