The keyboard shortcut associated with the Focus Editor item in the View menu is a quick way to set the keyboard focus to the main editor. Doing so makes the text cursor (blinking vertical bar) visible in the main editor allowing you to continue editing the file. The default keyboard shortcut is F9.
Using the Focus Editor keyboard shortcut allows you to quickly switch back and forth between the main editor and other panels. E.g. while working with the search and replace panel, you can press Tab on the keyboard to cycle through the various controls on the search and replace panel. To switch to the main editor, press F9. To switch back to the search and replace panel, press Ctrl+F.
If you have split the editor, then you can use this keyboard shortcut to switch keyboard focus between the two halves of the editor when one of them already has focus. When another part of EditPad has focus, this keyboard shortcut moves focus back to the editor half that had focus most recently.