This code you posted comes from the window for entering savegame description. The xResult and xDescription properties are used as "parameters" of the window, i.e. the script calling the window first fills-in the DescWindow.xDescription, then displays the window. And after the player closes the window, the DescWindow.xResult property is filled with "true" if he clicked "OK".
These preudo-properties are NOT related to the editor control and they are NOT standard WME properties. They are custom properties only used in this single script. That's why you won't find them in the documentation.
But back to your question, you simply set the text in the editor control by calling:
Editor.Text = "some text";
Or if you want to empty the editor, simply call:
Editor.Text = "";
And, of course, you read the text the other way around:
var SomeVariable = Editor.Text;
There's nothing complicated about it.