This feature is about having a built-in filemanager (usually much like XTree? or NortonCommander? or WindowsExplorer?) that allows you to remain in the relative comfort of the text editor and still manipulate files. GNU Emacs and Xemacs has a file manager called dired and also other filemangers are available for them.
Sometimes filemanagers have some kind of built-in text editor. For example, TkDesk?, Norton Commander, Connect (The IBM Handshaker), Midnight Commander (mcedit) and Unix Cockpit have built-in text editors.
Sometimes an already existing text editor is used as text editing component inside some bigger application. For example, that way it is possible at use Vim as text editor inside an E-Mail client called Evolution. Therefore it would be possible to use text editor as text editing component inside a filemanger, too.
This was probably more important with TextMode editors.